Get VideoCapturer stats via VideoTrackSourceInterface in StatsCollector,

webrtc/api/videotracksource.h

Index: webrtc/api/videotracksource.h
diff --git a/webrtc/api/videotracksource.h b/webrtc/api/videotracksource.h
index 78577aaba23b1f4e4efe345a3cf5fd4e875c8b28..d55679d6da55e31cb955a12047a97f579f3e1c4e 100644
--- a/webrtc/api/videotracksource.h
+++ b/webrtc/api/videotracksource.h
@@ -39,6 +39,8 @@ class VideoTrackSource : public Notifier<VideoTrackSourceInterface> {
   virtual bool is_screencast() const { return false; }
   virtual bool needs_denoising() const { return false; }
 
+  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;