Introduce WorkerBackingThread

third_party/WebKit/Source/core/workers/SharedWorkerThread.h

Index: third_party/WebKit/Source/core/workers/SharedWorkerThread.h
diff --git a/third_party/WebKit/Source/core/workers/SharedWorkerThread.h b/third_party/WebKit/Source/core/workers/SharedWorkerThread.h
index 28d7a217d2b91fd20ba5dfbfdac95d16bca99c69..f448aab1faba2ea9276b97214cfd40ef5f050d2d 100644
--- a/third_party/WebKit/Source/core/workers/SharedWorkerThread.h
+++ b/third_party/WebKit/Source/core/workers/SharedWorkerThread.h
@@ -43,15 +43,16 @@ public:
     static PassOwnPtr<SharedWorkerThread> create(const String& name, PassRefPtr<WorkerLoaderProxy>, WorkerReportingProxy&);
     ~SharedWorkerThread() override;
 
+    WorkerBackingThread& workerBackingThread() override { return *m_workerBackingThread; }
+
 protected:
     RawPtr<WorkerGlobalScope> createWorkerGlobalScope(PassOwnPtr<WorkerThreadStartupData>) override;
-    WebThreadSupportingGC& backingThread() override;
 
 private:
     SharedWorkerThread(const String& name, PassRefPtr<WorkerLoaderProxy>, WorkerReportingProxy&);
 
+    OwnPtr<WorkerBackingThread> m_workerBackingThread;
     String m_name;
-    OwnPtr<WebThreadSupportingGC> m_thread;
 };
 
 } // namespace blink