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 6d23d2e8cace164a949e923482f148e6c6bc3293..108209dc2c108cceacad3c081ca9bc9f9a72492d 100644
--- a/webrtc/api/videotracksource.h
+++ b/webrtc/api/videotracksource.h
@@ -40,6 +40,8 @@ class VideoTrackSource : public Notifier<VideoTrackSourceInterface> {
   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;