Restoring behavior where PeerConnection tracks changes to MediaStreams.

talk/app/webrtc/mediastreamobserver.h

 /*
  * libjingle
  * Copyright 2015 Google Inc.
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  *  1. Redistributions of source code must retain the above copyright notice,
  *     this list of conditions and the following disclaimer.
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  *     this list of conditions and the following disclaimer in the documentation
  *     and/or other materials provided with the distribution.
  *  3. The name of the author may not be used to endorse or promote products
  *     derived from this software without specific prior written permission.
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  * THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR IMPLIED
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-// This file is currently stubbed so that Chromium's build files can be updated.
-
 #ifndef TALK_APP_WEBRTC_MEDIASTREAMOBSERVER_H_
 #define TALK_APP_WEBRTC_MEDIASTREAMOBSERVER_H_
 
+#include "talk/app/webrtc/mediastreaminterface.h"
+#include "webrtc/base/scoped_ref_ptr.h"
+#include "webrtc/base/sigslot.h"
+
+namespace webrtc {
+
+// Helper class which will listen for changes to a stream and emit the
+// corresponding signals.
+class MediaStreamObserver : public ObserverInterface {
+ public:
+  explicit MediaStreamObserver(MediaStreamInterface* stream);
+  ~MediaStreamObserver();
+
+  const MediaStreamInterface* stream() const { return stream_; }
+
+  void OnChanged() override;
+
+  sigslot::signal2<AudioTrackInterface*, MediaStreamInterface*>
+      SignalAudioTrackAdded;
+  sigslot::signal2<AudioTrackInterface*, MediaStreamInterface*>
+      SignalAudioTrackRemoved;
+  sigslot::signal2<VideoTrackInterface*, MediaStreamInterface*>
+      SignalVideoTrackAdded;
+  sigslot::signal2<VideoTrackInterface*, MediaStreamInterface*>
+      SignalVideoTrackRemoved;
+
+ private:
+  rtc::scoped_refptr<MediaStreamInterface> stream_;
+  AudioTrackVector cached_audio_tracks_;
+  VideoTrackVector cached_video_tracks_;
+};
+
+}  // namespace webrtc
+
 #endif  // TALK_APP_WEBRTC_MEDIASTREAMOBSERVER_H_