[Media] Record time it takes to start rendering audio and video

media/blink/webmediaplayer_impl.cc

Index: media/blink/webmediaplayer_impl.cc
diff --git a/media/blink/webmediaplayer_impl.cc b/media/blink/webmediaplayer_impl.cc
index 654ba89050299e2a31ec49243c759482815edc6c..2a8874e02ce1bfcd3144c5b75af0abcdb38204d1 100644
--- a/media/blink/webmediaplayer_impl.cc
+++ b/media/blink/webmediaplayer_impl.cc
@@ -1340,6 +1340,11 @@ void WebMediaPlayerImpl::OnShown() {
   if (watch_time_reporter_)
     watch_time_reporter_->OnShown();
 
+  compositor_task_runner_->PostTask(
+      FROM_HERE,
+      base::Bind(&VideoFrameCompositor::SetShownTimestamp,
+                 base::Unretained(compositor_), base::TimeTicks::Now()));
+
   if (IsBackgroundVideoTrackOptimizationEnabled() &&
       client_->hasSelectedVideoTrack()) {
     WebMediaPlayer::TrackId trackId = client_->getSelectedVideoTrackId();