Clear DeferredTaskHandler's audio thread ID upon the thread going away.

third_party/WebKit/Source/modules/webaudio/DeferredTaskHandler.h

 /*
  * Copyright (C) 2010, Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1.  Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2.  Redistributions in binary form must reproduce the above copyright
  *    notice, this list of conditions and the following disclaimer in the
@@ skipping 83 matching lines @@
 
     void addRenderingOrphanHandler(PassRefPtr<AudioHandler>);
     void requestToDeleteHandlersOnMainThread();
     void clearHandlersToBeDeleted();
 
     //
     // Thread Safety and Graph Locking:
     //
     void setAudioThreadToCurrentThread();
     ThreadIdentifier audioThread() const { return acquireLoad(&m_audioThread); }
+    void clearAudioThread();
 
     // TODO(hongchan): Use no-barrier load here. (crbug.com/247328)
     //
     // It is okay to use a relaxed (no-barrier) load here. Because the data
     // referenced by m_audioThread is not actually being used, thus we do not
     // need a barrier between the load of m_audioThread and of that data.
     bool isAudioThread() const { return currentThread() == acquireLoad(&m_audioThread); }
 
     void lock();
     bool tryLock();
@@ skipping 73 matching lines @@
     Vector<RefPtr<AudioHandler>> m_deletableOrphanHandlers;
 
     // Graph locking.
     RecursiveMutex m_contextGraphMutex;
     volatile ThreadIdentifier m_audioThread;
 };
 
 } // namespace blink
 
 #endif // DeferredTaskHandler_h