Get VideoCapturer stats via VideoTrackSourceInterface in StatsCollector,

webrtc/api/videocapturertracksource.h

Index: webrtc/api/videocapturertracksource.h
diff --git a/webrtc/api/videocapturertracksource.h b/webrtc/api/videocapturertracksource.h
index 9a8d05d9ed9452ac2bac356f59f1c948bccb1fbc..c4aa3a90d7c8e9ae7e2e2f60ad3bd2645fdcd7c6 100644
--- a/webrtc/api/videocapturertracksource.h
+++ b/webrtc/api/videocapturertracksource.h
@@ -56,6 +56,8 @@ class VideoCapturerTrackSource : public VideoTrackSource,
   }
   bool needs_denoising() const override { return needs_denoising_; }
 
+  bool GetStats(Stats* stats) override;
+
   void Stop() override;
   void Restart() override;