Expose video freeze metrics in GetStats.

This adds the following non-standardized metrics to video receiver
stats:
- freezeCount
- pauseCount
- totalFreezesDuration
- totalPausesDuration
- totalFramesDuration
- sumOfSquaredFrameDurations

For description of these metrics see
https://henbos.github.io/webrtc-provisional-stats/#RTCVideoReceiverStats-dict*

Bug: webrtc:10145
Change-Id: I4c76d5651102e73b1592ffed561e6224f2badeb6
Reviewed-on: https://webrtc-review.googlesource.com/c/114840
Reviewed-by: Stefan Holmer <stefan@webrtc.org>
Reviewed-by: Henrik Boström <hbos@webrtc.org>
Reviewed-by: Ilya Nikolaevskiy <ilnik@webrtc.org>
Commit-Queue: Sergey Silkin <ssilkin@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#26523}
diff --git a/media/base/media_channel.h b/media/base/media_channel.h
index a55c191..d20de75 100644
--- a/media/base/media_channel.h
+++ b/media/base/media_channel.h
@@ -533,6 +533,12 @@
   uint32_t frames_rendered = 0;
   absl::optional<uint64_t> qp_sum;
   int64_t interframe_delay_max_ms = -1;
+  uint32_t freeze_count = 0;
+  uint32_t pause_count = 0;
+  uint32_t total_freezes_duration_ms = 0;
+  uint32_t total_pauses_duration_ms = 0;
+  uint32_t total_frames_duration_ms = 0;
+  double sum_squared_frame_durations = 0.0;
 
   webrtc::VideoContentType content_type = webrtc::VideoContentType::UNSPECIFIED;