Disallow cross-thread Persistent<> read access.

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

 /*
  * Copyright (C) 2012, 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 40 matching lines @@
 
   unsigned long maxChannelCount() const;
 
  private:
   MediaStreamAudioDestinationHandler(AudioNode&, size_t numberOfChannels);
   // As an audio source, we will never propagate silence.
   bool propagatesSilence() const override { return false; }
 
   // This Persistent doesn't make a reference cycle.
   Persistent<MediaStream> m_stream;
-  Persistent<MediaStreamSource> m_source;
+  // Accessed by main thread and during audio thread processing.
+  //
+  // TODO: try to avoid such access during audio thread processing.
+  CrossThreadPersistent<MediaStreamSource> m_source;
 
   // This synchronizes dynamic changes to the channel count with
   // process() to manage the mix bus.
   mutable Mutex m_processLock;
 
   // This internal mix bus is for up/down mixing the input to the actual
   // number of channels in the destination.
   RefPtr<AudioBus> m_mixBus;
 };
 
@@ skipping 10 matching lines @@
 
   MediaStream* stream() const;
 
  private:
   MediaStreamAudioDestinationNode(BaseAudioContext&, size_t numberOfChannels);
 };
 
 }  // namespace blink
 
 #endif  // MediaStreamAudioDestinationNode_h