compositor-worker: Get the thread to run compositor-workers from the Platform.

Source/core/workers/DedicatedWorkerThread.h

 /*
  * Copyright (C) 2009 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
  * met:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 29 matching lines @@
 
 class DedicatedWorkerThread final : public WorkerThread {
 public:
     static PassRefPtr<DedicatedWorkerThread> create(PassRefPtr<WorkerLoaderProxy>, WorkerObjectProxy&, double timeOrigin);
     WorkerObjectProxy& workerObjectProxy() const { return m_workerObjectProxy; }
     ~DedicatedWorkerThread() override;
 
 protected:
     PassRefPtrWillBeRawPtr<WorkerGlobalScope> createWorkerGlobalScope(PassOwnPtr<WorkerThreadStartupData>) override;
     void postInitialize() override;
-    WebThreadSupportingGC& backingThread() override;
+    WebThread& backingThread() override;
 
 private:
     DedicatedWorkerThread(PassRefPtr<WorkerLoaderProxy>, WorkerObjectProxy&, double timeOrigin);
 
     WorkerObjectProxy& m_workerObjectProxy;
     double m_timeOrigin;
     OwnPtr<WebThreadSupportingGC> m_thread;
 };
 
 } // namespace blink
 
 #endif // DedicatedWorkerThread_h