Guarantee a thread safety when accessing source location info in pannerNode.

Source/modules/webaudio/PannerNode.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 158 matching lines @@
     double m_cachedDopplerRate;
 
     // Cached listener parameters after processing.
     RefPtr<AudioListener> m_cachedListener;
 
     RefPtr<HRTFDatabaseLoader> m_hrtfDatabaseLoader;
 
     // AudioContext's connection count
     unsigned m_connectionCount;
 
-    // Synchronize process() and setPanningModel() which can change the panner.
-    mutable Mutex m_pannerLock;
+    // Synchronize process() with setting of the panning model, distance model and caching of the source location/orientation info.
+    mutable Mutex m_processLock;
 };
 
 } // namespace WebCore
 
 #endif // PannerNode_h