Change the order of animation layers in the mecanim.

@mouurusai's script didnt work anymore, please see an updated version below. The methods where not available anymore, so i use reflection to call them anyway. Absolutely mindblowing this is not standard behaviour.

 using UnityEditor;
 using UnityEditorInternal;
 using UnityEngine;
 
 public class AnimatorSwapLayers : EditorWindow
 {
     private Vector2 _scrollPosition;
 
     [MenuItem("Window/Animator Swap Layers")]
     static void Init()
     {
         AnimatorSwapLayers window = (AnimatorSwapLayers)GetWindow(typeof(AnimatorSwapLayers));
     }
 
 
     void OnSelectionChange()
     {
         Repaint();
     }
 
 
     void OnGUI()
     {
 
         if (Selection.objects.Length > 0 && Selection.objects[0] is AnimatorController)
         {
             AnimatorController controller = (AnimatorController)Selection.objects[0];
 
             _scrollPosition = GUILayout.BeginScrollView(_scrollPosition);
             for (int i = 0; i < controller.layerCount; i++)
             {
                 GUILayout.BeginHorizontal();
                 GUILayout.Label(controller.GetLayer(i).name);
                 GUILayout.FlexibleSpace();
                 if (i > 0)
                 {
                     if (GUILayout.Button("↑"))
                     {
                         Swap(controller, i, true);
                     }
                 }
 
                 if (i < controller.layerCount - 1)
                 {
                     if (GUILayout.Button("↓"))
                     {
                         Swap(controller, i, false);
                     }
                 }
                 GUILayout.EndHorizontal();
                 GUILayout.Space(10);
             }
             GUILayout.EndScrollView();
         }
         else if (Selection.objects.Length > 1)
         {
             GUILayout.Label("Multy edit not supported");
         }
         else
         {
             GUILayout.Label("No controller selected");
         }
     }
 
     void Swap(AnimatorController controller, int pos, bool moveUp)
     {
         int newPos = pos + (moveUp ? -1 : 1);
 
         AnimatorControllerLayer newLayer = controller.GetLayer(newPos),
             layer = controller.GetLayer(pos);
 
         AnimatorLayerBlendingMode blendingModeT = newLayer.blendingMode;
         bool iKPassT = newLayer.iKPass;
         AvatarMask layerMaskT = newLayer.avatarMask;
         string nameT = newLayer.name;
         StateMachine stateMachineT = newLayer.stateMachine;
         int syncedIndexT = newLayer.syncedLayerIndex;
 
         callPrivateMethod(controller, "SetLayerBlendingMode", newPos, layer.blendingMode);
         callPrivateMethod(controller, "SetLayerIKPass", newPos, layer.iKPass);
         callPrivateMethod(controller, "SetLayerMask", newPos, layer.avatarMask);
         callPrivateMethod(controller, "SetLayerName", newPos, layer.name);
         callPrivateMethod(controller, "SetLayerStateMachine", newPos, layer.stateMachine);
         callPrivateMethod(controller, "SetLayerSyncedIndex", newPos, layer.syncedLayerIndex);
 
         callPrivateMethod(controller, "SetLayerBlendingMode", pos, blendingModeT);
         callPrivateMethod(controller, "SetLayerIKPass", pos, iKPassT);
         callPrivateMethod(controller, "SetLayerMask", pos, layerMaskT);
         callPrivateMethod(controller, "SetLayerName", pos, nameT);
         callPrivateMethod(controller, "SetLayerStateMachine", pos, stateMachineT);
         callPrivateMethod(controller, "SetLayerSyncedIndex", pos, syncedIndexT);
     }
 
     private object callPrivateMethod(object obj, string methodname, params object[] args)
     {
         var method = obj.GetType().GetMethod(methodname, System.Reflection.BindingFlags.Instance | System.Reflection.BindingFlags.NonPublic);
 
         if (method == null)
             Debug.LogError("Couldnt find method!");
 
         return method.Invoke(obj, args);
     }
 
 }

Seems is work fine, but why, this absolutely obviously the must have function, is not implemented by default?

 using UnityEditor;
 using UnityEditorInternal;
 using UnityEngine;
 
 public class AnimatorSwapLayers : EditorWindow
 {
     private Vector2 _scrollPosition;
 
     [MenuItem("Window/Animator Swap Layers")]
     static void Init ()
     {
         AnimatorSwapLayers window = (AnimatorSwapLayers)GetWindow(typeof(AnimatorSwapLayers));
     }
 
 
     void OnSelectionChange()
     {
         Repaint();
     }
 
 
      void OnGUI()
      {
 
          if(Selection.objects.Length > 0 && Selection.objects[0] is AnimatorController)
         {
             AnimatorController controller = (AnimatorController)Selection.objects[0];
 
             _scrollPosition = GUILayout.BeginScrollView(_scrollPosition);
                 for (int i = 0; i < controller.GetLayerCount(); i++)
                 {
                     GUILayout.BeginHorizontal();
                         GUILayout.Label(controller.GetLayerName(i));
                             GUILayout.FlexibleSpace();
                         if(i>0)
                         {
                             if (GUILayout.Button("↑"))
                             {
                                 Swap(controller, i, true);
                             }
                         }
 
                         if (i < controller.GetLayerCount()-1)
                         {
                             if (GUILayout.Button("↓"))
                             {
                                 Swap(controller, i, false);
                             }
                         }
                     GUILayout.EndHorizontal();
                         GUILayout.Space(10);
                 }
             GUILayout.EndScrollView();
         }
         else if (Selection.objects.Length > 1)
         {
             GUILayout.Label("Multy edit not supported");
         }
         else
         {
             GUILayout.Label("No controller selected");
         }
     }
 
      void Swap(AnimatorController controller, int pos, bool moveUp)
      {
          int newPos = pos + (moveUp ? -1 : 1);
 
          AnimatorLayerBlendingMode blendingModeT = controller.GetLayerBlendingMode(newPos);
          bool iKPassT = controller.GetLayerIKPass(newPos);
          AvatarMask layerMaskT = controller.GetLayerMask(newPos);
          string nameT = controller.GetLayerName(newPos);
          StateMachine stateMachineT = controller.GetLayerStateMachine(newPos);
          int syncedIndexT = controller.GetLayerSyncedIndex(newPos);
          int layerTypeT = controller.GetLayerType(newPos);
 
 
          controller.SetLayerBlendingMode(newPos, controller.GetLayerBlendingMode(pos));
          controller.SetLayerIKPass(newPos, controller.GetLayerIKPass(pos));
          controller.SetLayerMask(newPos, controller.GetLayerMask(pos));
          controller.SetLayerName(newPos, controller.GetLayerName(pos));
          controller.SetLayerStateMachine(newPos, controller.GetLayerStateMachine(pos));
          controller.SetLayerSyncedIndex(newPos, controller.GetLayerSyncedIndex(pos));
          controller.SetLayerType(newPos, controller.GetLayerType(pos));
 
 
          controller.SetLayerBlendingMode(pos, blendingModeT);
          controller.SetLayerIKPass(pos, iKPassT);
          controller.SetLayerMask(pos, layerMaskT);
          controller.SetLayerName(pos, nameT);
          controller.SetLayerStateMachine(pos, stateMachineT);
          controller.SetLayerSyncedIndex(pos, syncedIndexT);
          controller.SetLayerType(pos, layerTypeT);
      }
 
 }