compositor-worker: Add mutable attributes to CompositorProxy.

Source/core/dom/CompositorProxy.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.
 
 [
     Constructor(Element element, DOMString[] attributeArray),
     ConstructorCallWith=ExecutionContext,
     Exposed=Window,
     GarbageCollected,
     RaisesException=Constructor,
     // RuntimeEnabled=CompositorWorker,
 ] interface CompositorProxy {
     readonly attribute long elementId;
     readonly attribute long attributes;
+
+    [RaisesException] attribute double opacity;
+    [RaisesException] attribute double scrollTop;
+    [RaisesException] attribute double[] transform;
 };