compositor-worker: Get the thread to run compositor-workers from the Platform.

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 a4c57754c1ee1b7f7a9f82ae2900ec319c0d9df3..973397543c7c4d83cde7b44bf71dbec797d5aec6 100644
--- a/Source/core/workers/WorkerThread.h
+++ b/Source/core/workers/WorkerThread.h
@@ -68,7 +68,7 @@ public:
     // a new thread on the first call (e.g. shared, dedicated workers), whereas
     // some implementations can use an existing thread that is already being
     // used by other workers (e.g.  compositor workers).
-    virtual WebThreadSupportingGC& backingThread() = 0;
+    virtual WebThread& backingThread() = 0;
 
     virtual void didStartRunLoop();
     virtual void didStopRunLoop();
@@ -150,7 +150,6 @@ private:
     void shutdown();
     void performShutdownTask();
     void performIdleWork(double deadlineSeconds);
-    void postDelayedTask(const WebTraceLocation&, PassOwnPtr<ExecutionContextTask>, long long delayMs);
 
     bool m_started;
     bool m_terminated;
@@ -170,6 +169,7 @@ private:
 
     RefPtrWillBePersistent<WorkerGlobalScope> m_workerGlobalScope;
 
+    OwnPtr<GCSupportForWebThread> m_gcSupport;
     v8::Isolate* m_isolate;
 
     // Used to signal thread shutdown.