compositor-worker: Share a thread and an isolate for compositor workers.

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 46 matching lines @@
     return SharedWorkerGlobalScope::create(m_name, this, startupData);
 }
 
 WebThreadSupportingGC& SharedWorkerThread::backingThread()
 {
     if (!m_thread)
         m_thread = WebThreadSupportingGC::create("SharedWorker Thread");
     return *m_thread.get();
 }
 
+PassOwnPtr<WorkerIsolateWrapper> SharedWorkerThread::createIsolateWrapper()
+{
+    return WorkerIsolateWrapper::createDefault();
+}
+
 } // namespace blink