compositor-worker: Introduce CompositorWorker.

Source/core/workers/WorkerMessagingProxy.h

Index: Source/core/workers/WorkerMessagingProxy.h
diff --git a/Source/core/workers/WorkerMessagingProxy.h b/Source/core/workers/WorkerMessagingProxy.h
index bace41ad093c60865366dcc47d3e175ddce6663b..c3fdd7f6d454ea34a51ba7c236d2975ea08b06b7 100644
--- a/Source/core/workers/WorkerMessagingProxy.h
+++ b/Source/core/workers/WorkerMessagingProxy.h
@@ -46,7 +46,7 @@ class Worker;
 class WorkerClients;
 class WorkerInspectorProxy;
 
-class WorkerMessagingProxy final
+class WorkerMessagingProxy
     : public WorkerGlobalScopeProxy
     , private WorkerLoaderProxyProvider {
     WTF_MAKE_NONCOPYABLE(WorkerMessagingProxy);
@@ -80,6 +80,11 @@ public:
 protected:
     virtual ~WorkerMessagingProxy();
 
+    virtual PassRefPtr<DedicatedWorkerThread> createWorkerThread(double originTime, PassOwnPtrWillBeRawPtr<WorkerThreadStartupData>);
+
+    PassRefPtr<WorkerLoaderProxy> loaderProxy() { return m_loaderProxy; }
+    WorkerObjectProxy& workerObjectProxy() { return *m_workerObjectProxy.get(); }
+
 private:
     static void workerObjectDestroyedInternal(ExecutionContext*, WorkerMessagingProxy*);
     void terminateInternally();