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

Source/core/workers/WorkerThread.h

Index: Source/core/workers/WorkerThread.h
diff --git a/Source/core/workers/WorkerThread.h b/Source/core/workers/WorkerThread.h
index ec75db5f9771fe8435ff70df53d9a1bb7796e3ed..0753bc3b766c0ec862f00feaafe5d1c87020077d 100644
--- a/Source/core/workers/WorkerThread.h
+++ b/Source/core/workers/WorkerThread.h
@@ -121,10 +121,6 @@ protected:
 
     virtual void postInitialize() { }
 
-    // Both of these methods are called in the worker thread.
-    virtual void initializeBackingThread();
-    virtual void shutdownBackingThread();
-
     virtual v8::Isolate* initializeIsolate();
     virtual void willDestroyIsolate();
     virtual void destroyIsolate();