workers: Move ownership of WebThread from WorkerThread

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/WorkerThread.h"
 
 namespace blink {
 
 class WorkerObjectProxy;
 
 class CompositorWorkerThread final : public WorkerThread {
 public:
     static PassRefPtr<CompositorWorkerThread> create(PassRefPtr<WorkerLoaderProxy>, WorkerObjectProxy&, double timeOrigin, PassOwnPtr<WorkerThreadStartupData>);
     virtual ~CompositorWorkerThread();
 
     WorkerObjectProxy& workerObjectProxy() const { return m_workerObjectProxy; }
 
 private:
     CompositorWorkerThread(PassRefPtr<WorkerLoaderProxy>, WorkerObjectProxy&, double timeOrigin, PassOwnPtr<WorkerThreadStartupData>);
 
     // WorkerThread:
     PassRefPtrWillBeRawPtr<WorkerGlobalScope> createWorkerGlobalScope(PassOwnPtr<WorkerThreadStartupData>) override;
+    WebThreadSupportingGC& backingThread() override;
 
     WorkerObjectProxy& m_workerObjectProxy;
     double m_timeOrigin;
+    OwnPtr<WebThreadSupportingGC> m_thread;
 };
 
 } // namespace blink
 
 #endif // CompositorWorkerThread_h