Get VideoCapturer stats via VideoTrackSourceInterface in StatsCollector,

webrtc/api/videocapturertracksource.h

 /*
  *  Copyright 2012 The WebRTC project authors. All Rights Reserved.
  *
  *  Use of this source code is governed by a BSD-style license
  *  that can be found in the LICENSE file in the root of the source
  *  tree. An additional intellectual property rights grant can be found
  *  in the file PATENTS.  All contributing project authors may
  *  be found in the AUTHORS file in the root of the source tree.
  */
 
@@ skipping 40 matching lines @@
     return video_capturer_.get();
   }
 
   bool is_screencast() const override {
     return video_capturer_->IsScreencast();
   }
   rtc::Optional<bool> needs_denoising() const override {
     return needs_denoising_;
   }
 
+  bool GetStats(Stats* stats) override;
+
   void Stop() override;
   void Restart() override;
 
  protected:
   VideoCapturerTrackSource(rtc::Thread* worker_thread,
                            cricket::VideoCapturer* capturer,
                            bool remote);
   virtual ~VideoCapturerTrackSource();
   void Initialize(const webrtc::MediaConstraintsInterface* constraints);
 
  private:
   void OnStateChange(cricket::VideoCapturer* capturer,
                      cricket::CaptureState capture_state);
 
   rtc::Thread* signaling_thread_;
   rtc::AsyncInvoker invoker_;
   rtc::scoped_ptr<cricket::VideoCapturer> video_capturer_;
   bool started_;
   cricket::VideoFormat format_;
   rtc::Optional<bool> needs_denoising_;
 };
 
 }  // namespace webrtc
 
 #endif  // WEBRTC_API_VIDEOCAPTURERTRACKSOURCE_H_