How to marshal a callback?

I'm trying to call a simple function with callback from JavaScript plugin. I have the following jslib file:

 var TestTimeout = {
     SetTimeout: function(callback, time) {
         setTimeout(callback, time);
     }
 };
 
 mergeInto(LibraryManager.library, TestTimeout);

The corresponding C# definition is

 public class TestTimeout {
     public delegate void CallbackDelegate();

     [DllImport("__Internal")]
     public static extern void SetTimeout(CallbackDelegate callback, int time);
 }

That gets called like

 void Start() {
     CSharp.SetTimeout(Callback, 3000);
 }

 [MonoPInvokeCallback(typeof(CSharp.CallbackDelegate))]
 private static void Callback() {
     Debug.Log("Test");
 }

But when I do so and build for WebGL target and run it from a simple server in Google Chrome 44, I can't recieve log message "Test", and even can't find logging call in code.

How do I properly call a JS function with callback?