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

Source/core/workers/SharedWorkerThread.h

Index: Source/core/workers/SharedWorkerThread.h
diff --git a/Source/core/workers/SharedWorkerThread.h b/Source/core/workers/SharedWorkerThread.h
index 9b52287459452f877626bd775c27bc34fe7547d5..1bc8e1d5c8e6d4210915020a969f98af2e7a5db7 100644
--- a/Source/core/workers/SharedWorkerThread.h
+++ b/Source/core/workers/SharedWorkerThread.h
@@ -45,13 +45,13 @@ public:
 
 protected:
     PassRefPtrWillBeRawPtr<WorkerGlobalScope> createWorkerGlobalScope(PassOwnPtr<WorkerThreadStartupData>) override;
-    WebThreadSupportingGC& backingThread() override;
+    WebThread& backingThread() override;
 
 private:
     SharedWorkerThread(const String& name, PassRefPtr<WorkerLoaderProxy>, WorkerReportingProxy&);
 
     String m_name;
-    OwnPtr<WebThreadSupportingGC> m_thread;
+    OwnPtr<WebThread> m_thread;
 };
 
 } // namespace blink