[CompositorWorker] Do not create WorkerBackingThread in shutdown sequence

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

 // 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.
 
 #include "modules/compositorworker/CompositorWorkerThread.h"
 
 #include "bindings/core/v8/V8GCController.h"
 #include "bindings/core/v8/V8Initializer.h"
 #include "core/workers/InProcessWorkerObjectProxy.h"
 #include "core/workers/WorkerBackingThread.h"
 #include "core/workers/WorkerThreadStartupData.h"
 #include "modules/compositorworker/CompositorWorkerGlobalScope.h"
 #include "platform/ThreadSafeFunctional.h"
 #include "platform/TraceEvent.h"
 #include "public/platform/Platform.h"
 
 namespace blink {
 
 namespace {
 
+// This is a singleton class holding the compositor worker thread in this
+// renderrer process. BackingThreadHolst::m_thread will never be cleared,
+// but Oilpan and V8 are detached from the thread when the last compositor
+// worker thread is gone.
+// See WorkerThread::terminateAndWaitForAllWorkers for the process shutdown
+// case.
 class BackingThreadHolder {
 public:
     static BackingThreadHolder& instance()
     {
         DEFINE_THREAD_SAFE_STATIC_LOCAL(BackingThreadHolder, holder, new BackingThreadHolder);
         return holder;
     }
 
     WorkerBackingThread* thread() { return m_thread.get(); }
-    void clear() { m_thread = nullptr; }
     void resetForTest()
     {
         ASSERT(!m_thread || (m_thread->workerScriptCount() == 0));
+        m_thread = nullptr;
         m_thread = WorkerBackingThread::createForTest(Platform::current()->compositorThread());
     }
 
 private:
     BackingThreadHolder() : m_thread(WorkerBackingThread::create(Platform::current()->compositorThread())) {}
 
     OwnPtr<WorkerBackingThread> m_thread;
 };
 
 } // namespace
@@ skipping 20 matching lines @@
 {
     return *BackingThreadHolder::instance().thread();
 }
 
 WorkerGlobalScope*CompositorWorkerThread::createWorkerGlobalScope(PassOwnPtr<WorkerThreadStartupData> startupData)
 {
     TRACE_EVENT0(TRACE_DISABLED_BY_DEFAULT("compositor-worker"), "CompositorWorkerThread::createWorkerGlobalScope");
     return CompositorWorkerGlobalScope::create(this, std::move(startupData), m_timeOrigin);
 }
 
-void CompositorWorkerThread::clearSharedBackingThread()
-{
-    BackingThreadHolder::instance().clear();
-}
-
 void CompositorWorkerThread::resetSharedBackingThreadForTest()
 {
     BackingThreadHolder::instance().resetForTest();
 }
 
 } // namespace blink