Add support for unmixed audio from remote WebRTC remote tracks.

content/renderer/media/media_stream_track.cc

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "content/renderer/media/media_stream_track.h"
 
 #include "base/logging.h"
 #include "third_party/WebKit/public/platform/WebMediaStreamSource.h"
 #include "third_party/libjingle/source/talk/app/webrtc/mediastreaminterface.h"
 
 namespace content {
 
 // static
 MediaStreamTrack* MediaStreamTrack::GetTrack(
     const blink::WebMediaStreamTrack& track) {
   if (track.isNull())
     return NULL;
   return static_cast<MediaStreamTrack*>(track.extraData());
 }
 
 MediaStreamTrack::MediaStreamTrack(bool is_local_track)
     : is_local_track_(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;
 }
 
 }  // namespace content