(POC) WebMessaging: Send a transferable ArrayBuffer over MessagePort by copy

Source/core/dom/MessagePort.idl

 /*
  * Copyright (C) 2008 Apple Inc. All Rights Reserved.
  * 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/#message-ports
 
 [
     ActiveDOMObject,
     Exposed=(Window,Worker),
     GarbageCollected,
 ] interface MessagePort : EventTarget {
-    [PostMessage, RaisesException, Measure] void postMessage(any message, optional sequence<Transferable> transfer);
+    [Custom, RaisesException, Measure] void postMessage(any message, optional sequence<Transferable> transfer);
     [Measure] void start();
     [Measure] void close();
 
     // event handlers
     attribute EventHandler onmessage;
 };