compositor-worker: Restrict the global interface in CompositorWorker.

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,
+    Exposed=(Window,CompositorWorker),
     GarbageCollected,
     RaisesException=Constructor,
     RuntimeEnabled=CompositorWorker,
 ] interface CompositorProxy {
     [RaisesException] attribute double opacity;
     [RaisesException] attribute double scrollLeft;
     [RaisesException] attribute double scrollTop;
     [RaisesException] attribute DOMMatrix transform;
 
     boolean supports(DOMString attribute);
     void disconnect();
 };