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

Source/modules/serviceworkers/ServiceWorkerThread.cpp

 /*
  * Copyright (C) 2013 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 45 matching lines @@
     return ServiceWorkerGlobalScope::create(this, startupData);
 }
 
 WebThreadSupportingGC& ServiceWorkerThread::backingThread()
 {
     if (!m_thread)
         m_thread = WebThreadSupportingGC::create("ServiceWorker Thread");
     return *m_thread.get();
 }
 
+OwnPtr<WorkerV8Isolate>& ServiceWorkerThread::workerIsolate()
+{
+    if (!m_isolate) {
+        ASSERT(isCurrentThread());
+        m_isolate = WorkerV8Isolate::createDefault();
+    }
+    return m_isolate;
+}
+
 } // namespace blink