Require use of AudioBus::create() to avoid ref-counting issues

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 44 matching lines @@
     MediaStreamAudioDestinationNode(AudioContext*, size_t numberOfChannels);
 
     virtual double tailTime() const OVERRIDE { return 0; }
     virtual double latencyTime() const OVERRIDE { return 0; }
 
     // As an audio source, we will never propagate silence.
     virtual bool propagatesSilence() const OVERRIDE { return false; }
 
     RefPtr<MediaStream> m_stream;
     RefPtr<MediaStreamSource> m_source;
-    AudioBus m_mixBus;
+    RefPtr<AudioBus> m_mixBus;
 };
 
 } // namespace WebCore
 
 #endif // ENABLE(WEB_AUDIO)
 
 #endif // MediaStreamAudioDestinationNode_h