compositor-worker: Allow registering compositor-frame callbacks.

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=(Worker,CompositorWorker),
     RuntimeEnabled=CompositorWorker,
     TypeChecking=Interface,
 ] interface CompositorWorkerGlobalScope : WorkerGlobalScope {
     [Custom, RaisesException] void postMessage(any message, optional sequence<Transferable> transfer);
     attribute EventHandler onmessage;
+
+    long requestAnimationFrame(FrameRequestCallback callback);
+    void cancelAnimationFrame(long handle);
 };