workers: Move ownership of WebThread from WorkerThread

Source/modules/serviceworkers/ServiceWorkerThread.h

Index: Source/modules/serviceworkers/ServiceWorkerThread.h
diff --git a/Source/modules/serviceworkers/ServiceWorkerThread.h b/Source/modules/serviceworkers/ServiceWorkerThread.h
index 2d750bb846d38f2f2714b7ff26e5b1eb46b02b06..4a30289d912652c8bc83659755bcdfdff29ba15c 100644
--- a/Source/modules/serviceworkers/ServiceWorkerThread.h
+++ b/Source/modules/serviceworkers/ServiceWorkerThread.h
@@ -44,10 +44,13 @@ public:
     virtual ~ServiceWorkerThread();
 
 protected:
-    virtual PassRefPtrWillBeRawPtr<WorkerGlobalScope> createWorkerGlobalScope(PassOwnPtr<WorkerThreadStartupData>) override;
+    PassRefPtrWillBeRawPtr<WorkerGlobalScope> createWorkerGlobalScope(PassOwnPtr<WorkerThreadStartupData>) override;
+    WebThreadSupportingGC& backingThread() override;
 
 private:
     ServiceWorkerThread(PassRefPtr<WorkerLoaderProxy>, WorkerReportingProxy&, PassOwnPtr<WorkerThreadStartupData>);
+
+    OwnPtr<WebThreadSupportingGC> m_thread;
 };
 
 } // namespace blink