compositor-worker: Create a separate CompositorWorkerGlobalScope for compositor workers.

Source/modules/compositorworker/CompositorWorkerThread.h

 // 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.
 
 #ifndef CompositorWorkerThread_h
 #define CompositorWorkerThread_h
 
 #include "core/workers/DedicatedWorkerThread.h"
 
 namespace blink {
 
 class CompositorWorkerThread final : public DedicatedWorkerThread {
 public:
     static PassRefPtr<CompositorWorkerThread> create(PassRefPtr<WorkerLoaderProxy>, WorkerObjectProxy&, double timeOrigin, PassOwnPtrWillBeRawPtr<WorkerThreadStartupData>);
     virtual ~CompositorWorkerThread();
 
 private:
     CompositorWorkerThread(PassRefPtr<WorkerLoaderProxy>, WorkerObjectProxy&, double timeOrigin, PassOwnPtrWillBeRawPtr<WorkerThreadStartupData>);
 
+    PassRefPtrWillBeRawPtr<WorkerGlobalScope> createWorkerGlobalScope(PassOwnPtrWillBeRawPtr<WorkerThreadStartupData>) override;
+
     v8::Isolate* initializeIsolate() override;
     void willDestroyIsolate() override;
     void destroyIsolate() override;
     void terminateV8Execution() override;
 
     PassOwnPtr<WebThreadSupportingGC> createWebThreadSupportingGC() override;
     void didStartRunLoop() override;
     void didStopRunLoop() override;
 };
 
 } // namespace blink
 
 #endif // CompositorWorkerThread_h