Report timing frames info in GetStats.

Some frames are already marked as 'timing frames' via video-timing RTP header extension. Timestamps along full WebRTC pipeline are gathered for these frames. This CL implements reporting of these timestamps for a single
timing frame since the last GetStats(). The frame with the longest end-to-end delay between two consecutive GetStats calls is reported.

The purpose of this timing information is not to provide a realtime statistics but to provide debugging information as it will help identify problematic places in video pipeline for outliers (frames which took longest to process).

BUG=webrtc:7594

Review-Url: https://codereview.webrtc.org/2946413002
Cr-Commit-Position: refs/heads/master@{#18909}
diff --git a/webrtc/pc/statscollector.cc b/webrtc/pc/statscollector.cc
index c932583..27cb6c3 100644
--- a/webrtc/pc/statscollector.cc
+++ b/webrtc/pc/statscollector.cc
@@ -249,6 +249,10 @@
   for (const auto& i : ints)
     report->AddInt(i.name, i.value);
   report->AddString(StatsReport::kStatsValueNameMediaType, "video");
+  if (info.timing_frame_info) {
+    report->AddString(StatsReport::kStatsValueNameTimingFrameInfo,
+                      info.timing_frame_info->ToString());
+  }
 }
 
 void ExtractStats(const cricket::VideoSenderInfo& info, StatsReport* report) {