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

Source/modules/webaudio/AudioContext.cpp

Index: Source/modules/webaudio/AudioContext.cpp
diff --git a/Source/modules/webaudio/AudioContext.cpp b/Source/modules/webaudio/AudioContext.cpp
index 508d8c941c023d35e407c291f9e1446681b15589..599113c3530c52d0c3471fa995abe5348f5f08f0 100644
--- a/Source/modules/webaudio/AudioContext.cpp
+++ b/Source/modules/webaudio/AudioContext.cpp
@@ -437,17 +437,18 @@ PassRefPtr<MediaStreamAudioSourceNode> AudioContext::createMediaStreamSource(Med
     AudioSourceProvider* provider = 0;
 
     MediaStreamTrackVector audioTracks = mediaStream->getAudioTracks();
+    RefPtr<MediaStreamTrack> audioTrack;
 
     // FIXME: get a provider for non-local MediaStreams (like from a remote peer).
     for (size_t i = 0; i < audioTracks.size(); ++i) {
-        RefPtr<MediaStreamTrack> localAudio = audioTracks[i];
-        if (localAudio->component()->audioSourceProvider()) {
-            provider = localAudio->component()->audioSourceProvider();
+        audioTrack = audioTracks[i];
+        if (audioTrack->component()->audioSourceProvider()) {
+            provider = audioTrack->component()->audioSourceProvider();
             break;
         }
     }
 
-    RefPtr<MediaStreamAudioSourceNode> node = MediaStreamAudioSourceNode::create(this, mediaStream, provider);
+    RefPtr<MediaStreamAudioSourceNode> node = MediaStreamAudioSourceNode::create(this, mediaStream, audioTrack.get(), provider);
 
     // FIXME: Only stereo streams are supported right now. We should be able to accept multi-channel streams.
     node->setFormat(2, sampleRate());