[CompositorWorker] Do not create WorkerBackingThread in shutdown sequence

third_party/WebKit/Source/modules/compositorworker/CompositorWorkerThread.h

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef CompositorWorkerThread_h
 #define CompositorWorkerThread_h
 
 #include "core/workers/WorkerThread.h"
 #include "modules/ModulesExport.h"
 
 namespace blink {
 
 class InProcessWorkerObjectProxy;
 
 // This class is overridden in unit-tests.
 class MODULES_EXPORT CompositorWorkerThread final : public WorkerThread {
 public:
     static PassOwnPtr<CompositorWorkerThread> create(PassRefPtr<WorkerLoaderProxy>, InProcessWorkerObjectProxy&, double timeOrigin);
     ~CompositorWorkerThread() override;
 
     InProcessWorkerObjectProxy& workerObjectProxy() const { return m_workerObjectProxy; }
     WorkerBackingThread& workerBackingThread() override;
     bool shouldAttachThreadDebugger() const override { return false; }
 
     static void resetSharedBackingThreadForTest();
-    static void clearSharedBackingThread();
 
 protected:
     CompositorWorkerThread(PassRefPtr<WorkerLoaderProxy>, InProcessWorkerObjectProxy&, double timeOrigin);
 
     WorkerGlobalScope* createWorkerGlobalScope(PassOwnPtr<WorkerThreadStartupData>) override;
 
 private:
     InProcessWorkerObjectProxy& m_workerObjectProxy;
     double m_timeOrigin;
 };
 
 } // namespace blink
 
 #endif // CompositorWorkerThread_h