Sync some event interfaces with the HTML spec

Source/core/events/MessageEvent.idl

 /*
  * Copyright (C) 2007 Henry Mason <hmason@mac.com>
  * Copyright (C) 2011 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2. Redistributions in binary form must reproduce the above copyright
@@ skipping 14 matching lines @@
  *
  */
 
 // https://html.spec.whatwg.org/multipage/comms.html#the-messageevent-interfaces
 
 [
     Constructor(DOMString type, optional MessageEventInit eventInitDict),
     Exposed=(Window,Worker),
     RaisesException=Constructor,
 ] interface MessageEvent : Event {
+    [Custom=Getter] readonly attribute any data;
     readonly attribute DOMString origin;
     readonly attribute DOMString lastEventId;
-    readonly attribute EventTarget? source; // May be a Window or a MessagePort
-    [Custom=Getter] readonly attribute any data;
+    // TODO(bashi): |source| should be (WindowProxy or MessagePort)?
+    readonly attribute EventTarget? source;
     readonly attribute MessagePort[]? ports;
+
+    // TODO(philipj): None of the initMessageEvent() arguments should be
+    // optional, and |sourceArg| and |portsArg| are of the wrong type.
     [Custom, MeasureAs=InitMessageEvent] void initMessageEvent([Default=Undefined] optional DOMString typeArg,
                                    [Default=Undefined] optional boolean canBubbleArg,
                                    [Default=Undefined] optional boolean cancelableArg,
                                    [Default=Undefined] optional any dataArg,
                                    [Default=Undefined] optional DOMString originArg,
                                    [Default=Undefined] optional DOMString lastEventIdArg,
                                    [Default=Undefined] optional Window sourceArg,
-                                   [Default=Undefined] optional MessagePort[] messagePorts);
+                                   [Default=Undefined] optional MessagePort[] portsArg);
 };