Add support for unmixed audio from remote WebRTC remote tracks.

content/renderer/media/media_stream_track.cc

Index: content/renderer/media/media_stream_track.cc
diff --git a/content/renderer/media/media_stream_track.cc b/content/renderer/media/media_stream_track.cc
index 70ebca194cc27d80e6016dc4eece417e3746b99b..7ce6d85057edb06f1d3f42cb4e02217f9cd184d6 100644
--- a/content/renderer/media/media_stream_track.cc
+++ b/content/renderer/media/media_stream_track.cc
@@ -25,7 +25,24 @@ MediaStreamTrack::MediaStreamTrack(bool is_local_track)
 MediaStreamTrack::~MediaStreamTrack() {
 }
 
-webrtc::AudioTrackInterface* MediaStreamTrack::GetAudioAdapter() {
+MediaStreamAudioTrack::MediaStreamAudioTrack(bool is_local_track)
+    : MediaStreamTrack(is_local_track) {
+}
+
+MediaStreamAudioTrack::~MediaStreamAudioTrack() {
+}
+
+// static
+MediaStreamAudioTrack* MediaStreamAudioTrack::GetTrack(
+    const blink::WebMediaStreamTrack& track) {
+  if (track.isNull() ||
+      track.source().type() != blink::WebMediaStreamSource::TypeAudio) {
+    return nullptr;
+  }
+  return static_cast<MediaStreamAudioTrack*>(track.extraData());
+}
+
+webrtc::AudioTrackInterface* MediaStreamAudioTrack::GetAudioAdapter() {
   NOTREACHED();
   return nullptr;
 }