Add support for unmixed audio from remote WebRTC remote tracks.

content/renderer/media/media_stream_audio_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_audio_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 {
+
+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