Fix potential thread safety issue on ScriptProcessNdoe during fire onaudioprocess eventhandler.

Source/modules/webaudio/ScriptProcessorNode.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 65 matching lines @@
     // Double buffering
     unsigned doubleBufferIndex() const { return m_doubleBufferIndex; }
     void swapBuffers() { m_doubleBufferIndex = 1 - m_doubleBufferIndex; }
     unsigned m_doubleBufferIndex;
     unsigned m_doubleBufferIndexForEvent;
     Vector<RefPtr<AudioBuffer> > m_inputBuffers;
     Vector<RefPtr<AudioBuffer> > m_outputBuffers;
 
     size_t m_bufferSize;
     unsigned m_bufferReadWriteIndex;
-    volatile bool m_isRequestOutstanding;
 
     unsigned m_numberOfInputChannels;
     unsigned m_numberOfOutputChannels;
 
     RefPtr<AudioBus> m_internalInputBus;
+
+    // Synchronize process() with fireProcessEvent().
+    mutable Mutex m_processEventLock;
 };
 
 } // namespace WebCore
 
 #endif // ScriptProcessorNode_h