Get VideoCapturer stats via VideoTrackSourceInterface in StatsCollector,

webrtc/api/videotracksource.h

 /*
  *  Copyright 2016 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 22 matching lines @@
   SourceState state() const override { return state_; }
   bool remote() const override { return remote_; }
 
   void Stop() override{};
   void Restart() override{};
 
   virtual bool is_screencast() const { return false; }
   virtual rtc::Optional<bool> needs_denoising() const {
     return rtc::Optional<bool>(); }
 
+  bool GetStats(Stats* stats) override { return false; }
+
   void AddOrUpdateSink(rtc::VideoSinkInterface<cricket::VideoFrame>* sink,
                        const rtc::VideoSinkWants& wants) override;
   void RemoveSink(rtc::VideoSinkInterface<cricket::VideoFrame>* sink) override;
 
   cricket::VideoCapturer* GetVideoCapturer() override { return nullptr; }
 
  protected:
   rtc::Thread* worker_thread() { return worker_thread_; }
 
  private:
   rtc::VideoSourceInterface<cricket::VideoFrame>* source_;
   rtc::Thread* worker_thread_;
   cricket::VideoOptions options_;
   SourceState state_;
   const bool remote_;
 };
 
 }  // namespace webrtc
 
 #endif  //  WEBRTC_API_VIDEOTRACKSOURCE_H_