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

Source/modules/webaudio/ScriptProcessorNode.h

Index: Source/modules/webaudio/ScriptProcessorNode.h
diff --git a/Source/modules/webaudio/ScriptProcessorNode.h b/Source/modules/webaudio/ScriptProcessorNode.h
index 515059fe0d4dc1db4647e27e224bdd8cabdebd8b..08a187df65576382d6bb8b5c43f893d02af020f8 100644
--- a/Source/modules/webaudio/ScriptProcessorNode.h
+++ b/Source/modules/webaudio/ScriptProcessorNode.h
@@ -83,12 +83,14 @@ private:
 
     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