Add periodic logging of video stats.

webrtc/video_send_stream.h

Index: webrtc/video_send_stream.h
diff --git a/webrtc/video_send_stream.h b/webrtc/video_send_stream.h
index 886367f0a50fb3d94b269a4208a1ed27be4d0828..084953a1a1284daa18de48991356134b3f50f56d 100644
--- a/webrtc/video_send_stream.h
+++ b/webrtc/video_send_stream.h
@@ -41,6 +41,8 @@ class VideoCaptureInput {
 class VideoSendStream {
  public:
   struct StreamStats {
+    std::string ToString() const;
+
     FrameCounts frame_counts;
     int width = 0;
     int height = 0;
@@ -55,6 +57,7 @@ class VideoSendStream {
   };
 
   struct Stats {
+    std::string ToString(int64_t time_ms) const;
     std::string encoder_implementation_name = "unknown";
     int input_frame_rate = 0;
     int encode_frame_rate = 0;