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

Source/modules/compositorworker/CompositorWorkerThread.h

Index: Source/modules/compositorworker/CompositorWorkerThread.h
diff --git a/Source/modules/compositorworker/CompositorWorkerThread.h b/Source/modules/compositorworker/CompositorWorkerThread.h
index 157c588d9d006513891b8bccaad15099688981ab..7c62b3b23fd2a1545b0c9e80fecffa164875f24f 100644
--- a/Source/modules/compositorworker/CompositorWorkerThread.h
+++ b/Source/modules/compositorworker/CompositorWorkerThread.h
@@ -11,32 +11,23 @@
 
 class WorkerObjectProxy;
 
-// This class is overridden in unit-tests.
-class CompositorWorkerThread : public WorkerThread {
+class CompositorWorkerThread final : public WorkerThread {
 public:
     static PassRefPtr<CompositorWorkerThread> create(PassRefPtr<WorkerLoaderProxy>, WorkerObjectProxy&, double timeOrigin);
     virtual ~CompositorWorkerThread();
 
     WorkerObjectProxy& workerObjectProxy() const { return m_workerObjectProxy; }
 
-protected:
+private:
     CompositorWorkerThread(PassRefPtr<WorkerLoaderProxy>, WorkerObjectProxy&, double timeOrigin);
 
     // WorkerThread:
     PassRefPtrWillBeRawPtr<WorkerGlobalScope> createWorkerGlobalScope(PassOwnPtr<WorkerThreadStartupData>) override;
     WebThreadSupportingGC& backingThread() override;
-    void didStartRunLoop() override { }
-    void didStopRunLoop() override { }
-    void initializeBackingThread() override;
-    void shutdownBackingThread() override;
-    v8::Isolate* initializeIsolate() override;
-    void willDestroyIsolate() override;
-    void destroyIsolate() override;
-    void terminateV8Execution() override;
 
-private:
     WorkerObjectProxy& m_workerObjectProxy;
     double m_timeOrigin;
+    OwnPtr<WebThreadSupportingGC> m_thread;
 };
 
 } // namespace blink