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

Source/core/workers/SharedWorkerThread.cpp

 /*
  * 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 39 matching lines @@
 
 SharedWorkerThread::~SharedWorkerThread()
 {
 }
 
 PassRefPtrWillBeRawPtr<WorkerGlobalScope> SharedWorkerThread::createWorkerGlobalScope(PassOwnPtr<WorkerThreadStartupData> startupData)
 {
     return SharedWorkerGlobalScope::create(m_name, this, startupData);
 }
 
-WebThreadSupportingGC& SharedWorkerThread::backingThread()
+WebThread& SharedWorkerThread::backingThread()
 {
     if (!m_thread)
-        m_thread = WebThreadSupportingGC::create("SharedWorker Thread");
-    return *m_thread.get();
+        m_thread = adoptPtr(Platform::current()->createThread("SharedWorker Thread"));
+    return *m_thread;
 }
 
 } // namespace blink