[bindings] Introduce PostMessage extended keyword

Source/modules/compositorworker/CompositorWorkerGlobalScope.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.
 
 [
     Exposed=CompositorWorker,
     Global=CompositorWorker,
     RuntimeEnabled=CompositorWorker,
     TypeChecking=Interface,
 ] interface CompositorWorkerGlobalScope : WorkerGlobalScope {
-    [Custom, RaisesException] void postMessage(any message, optional sequence<Transferable> transfer);
+    [Custom=PostMessageCommon, RaisesException] void postMessage(any message, optional sequence<Transferable> transfer);
     attribute EventHandler onmessage;
 
     long requestAnimationFrame(FrameRequestCallback callback);
     void cancelAnimationFrame(long handle);
 };