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

Source/core/workers/DedicatedWorkerThread.h

Index: Source/core/workers/DedicatedWorkerThread.h
diff --git a/Source/core/workers/DedicatedWorkerThread.h b/Source/core/workers/DedicatedWorkerThread.h
index b803c5baf6996f5663591de9d2768e6c8a68e0d5..e2d411d44e800b3213712b25786a88b7b1e76071 100644
--- a/Source/core/workers/DedicatedWorkerThread.h
+++ b/Source/core/workers/DedicatedWorkerThread.h
@@ -47,14 +47,14 @@ public:
 protected:
     PassRefPtrWillBeRawPtr<WorkerGlobalScope> createWorkerGlobalScope(PassOwnPtr<WorkerThreadStartupData>) override;
     void postInitialize() override;
-    WebThreadSupportingGC& backingThread() override;
+    WebThread& backingThread() override;
 
 private:
     DedicatedWorkerThread(PassRefPtr<WorkerLoaderProxy>, WorkerObjectProxy&, double timeOrigin);
 
     WorkerObjectProxy& m_workerObjectProxy;
     double m_timeOrigin;
-    OwnPtr<WebThreadSupportingGC> m_thread;
+    OwnPtr<WebThread> m_thread;
 };
 
 } // namespace blink