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 2405c86493da3a2d74d818d58207bc8e2333576c..16d35bade80e0227a30e0368ed36430118f32692 100644
--- a/Source/core/workers/WorkerThread.h
+++ b/Source/core/workers/WorkerThread.h
@@ -122,6 +122,10 @@ 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();