Get VideoCapturer stats via VideoTrackSourceInterface in StatsCollector,

webrtc/api/rtpsender.h

Index: webrtc/api/rtpsender.h
diff --git a/webrtc/api/rtpsender.h b/webrtc/api/rtpsender.h
index 879a332209b3bdef484da7094178005b36286107..3919e070dc579fe7e69d9df13199d4468cab2afe 100644
--- a/webrtc/api/rtpsender.h
+++ b/webrtc/api/rtpsender.h
@@ -101,6 +101,8 @@ class AudioRtpSender : public ObserverInterface,
   bool SetParameters(const RtpParameters& parameters);
 
  private:
+  // TODO(nisse): Since SSRC == 0 is technically valid, figure out
+  // some other way to test if we have a valid SSRC.
   bool can_send_track() const { return track_ && ssrc_; }
   // Helper function to construct options for
   // AudioProviderInterface::SetAudioSend.