Fixed the crash in the wbaudio source provider when the audio track is going away.

Source/modules/webaudio/MediaStreamAudioSourceNode.h

Index: Source/modules/webaudio/MediaStreamAudioSourceNode.h
diff --git a/Source/modules/webaudio/MediaStreamAudioSourceNode.h b/Source/modules/webaudio/MediaStreamAudioSourceNode.h
index 12d44ec297f9d80adc7d15c5154c9d583df6a02e..625690bc5e1bb6123ed20b0c521ee681d11f7ed7 100644
--- a/Source/modules/webaudio/MediaStreamAudioSourceNode.h
+++ b/Source/modules/webaudio/MediaStreamAudioSourceNode.h
@@ -41,7 +41,7 @@ class AudioContext;
 
 class MediaStreamAudioSourceNode : public AudioSourceNode, public AudioSourceProviderClient {
 public:
-    static PassRefPtr<MediaStreamAudioSourceNode> create(AudioContext*, MediaStream*, AudioSourceProvider*);
+    static PassRefPtr<MediaStreamAudioSourceNode> create(AudioContext*, MediaStream*, MediaStreamTrack*, AudioSourceProvider*);
 
     virtual ~MediaStreamAudioSourceNode();
 
@@ -57,12 +57,13 @@ public:
     AudioSourceProvider* audioSourceProvider() const { return m_audioSourceProvider; }
 
 private:
-    MediaStreamAudioSourceNode(AudioContext*, MediaStream*, AudioSourceProvider*);
+    MediaStreamAudioSourceNode(AudioContext*, MediaStream*, MediaStreamTrack*, AudioSourceProvider*);
 
     // As an audio source, we will never propagate silence.
     virtual bool propagatesSilence() const OVERRIDE { return false; }
 
     RefPtr<MediaStream> m_mediaStream;
+    RefPtr<MediaStreamTrack> m_audioTrack;
     AudioSourceProvider* m_audioSourceProvider;
 
     Mutex m_processLock;