[bindings] Use |PostMessage| attribute for CompositorWorker*.postMessage to eliminate custom bindin…

Source/modules/compositorworker/CompositorWorker.idl

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 [
     ActiveDOMObject,
     Constructor(DOMString scriptUrl),
     ConstructorCallWith=ExecutionContext,
     RaisesException=Constructor,
     RuntimeEnabled=CompositorWorker,
     WillBeGarbageCollected,
 ] interface CompositorWorker : EventTarget {
     attribute EventHandler onmessage;
-    [Custom, RaisesException] void postMessage(SerializedScriptValue message, optional sequence<Transferable> transfer);
+    [PostMessage, RaisesException] void postMessage(SerializedScriptValue message, optional sequence<Transferable> transfer);
     void terminate();
 };
 
 CompositorWorker implements AbstractWorker;