compositor-worker: Keep worker backing thread alive for the lifetime of the compositor thread.

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

 // Copyright 2016 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 WorkerBackingThread_h
 #define WorkerBackingThread_h
 
 #include "core/CoreExport.h"
 #include "wtf/Forward.h"
 #include "wtf/OwnPtr.h"
 #include "wtf/PassOwnPtr.h"
 #include "wtf/ThreadingPrimitives.h"
 #include <v8.h>
 
 namespace blink {
 
+class WaitableEvent;
 class WebThread;
 class WebThreadSupportingGC;
 
 // WorkerBackingThread represents a WebThread with Oilpan and V8 potentially
 // shared by multiple WebWorker scripts. A WebWorker needs to call attach() when
 // attaching itself to the backing thread, and call detach() when the script
 // no longer needs the thread.
 // A WorkerBackingThread represents a WebThread while a WorkerThread corresponds
 // to a web worker. There is one-to-one correspondence between WorkerThread and
 // WorkerGlobalScope, but multiple WorkerThreads (i.e., multiple
@@ skipping 31 matching lines @@
         DCHECK(m_backingThread);
         return *m_backingThread;
     }
 
     v8::Isolate* isolate() { return m_isolate; }
 
 private:
     WorkerBackingThread(const char* name, bool shouldCallGCOnShutdown);
     WorkerBackingThread(WebThread*, bool shouldCallGCOnSHutdown);
     void initialize();
-    void shutdown();
+    void shutdown(WaitableEvent* doneEvent = nullptr);
+    void signalShutdownAndWait();
 
     // Protects |m_workerScriptCount|.
     Mutex m_mutex;
     OwnPtr<WebThreadSupportingGC> m_backingThread;
     v8::Isolate* m_isolate = nullptr;
     unsigned m_workerScriptCount = 0;
     bool m_isOwningThread;
     bool m_shouldCallGCOnShutdown;
 };
 
 } // namespace blink
 
 #endif // WorkerBackingThread_h