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

media/renderers/video_renderer_impl.cc

Index: media/renderers/video_renderer_impl.cc
diff --git a/media/renderers/video_renderer_impl.cc b/media/renderers/video_renderer_impl.cc
index 2c13377bd0519218571104f9a3559dbc8932ca14..2a762fc24afd727fd18ae3be4c0e408ba9c25386 100644
--- a/media/renderers/video_renderer_impl.cc
+++ b/media/renderers/video_renderer_impl.cc
@@ -129,6 +129,7 @@ void VideoRendererImpl::StartPlayingFrom(base::TimeDelta timestamp) {
   state_ = kPlaying;
   start_timestamp_ = timestamp;
   painted_first_frame_ = false;
+  start_rendering_time_ = tick_clock_->NowTicks();
   AttemptRead_Locked();
 }
 
@@ -210,6 +211,7 @@ scoped_refptr<VideoFrame> VideoRendererImpl::Render(
   if (!background_rendering && !was_background_rendering_)
     frames_dropped_ += frames_dropped;
   UpdateStats_Locked();
+
   was_background_rendering_ = background_rendering;
 
   // Always post this task, it will acquire new frames if necessary and since it
@@ -422,6 +424,13 @@ void VideoRendererImpl::FrameReady(VideoFrameStream::Status status,
                               first_frame->natural_size());
       sink_->PaintSingleFrame(first_frame);
       painted_first_frame_ = true;
+
+      if (!start_rendering_time_.is_null()) {
+        UMA_HISTOGRAM_MEDIUM_TIMES(
+            "Media.Video.FirstRenderTime",
+            tick_clock_->NowTicks() - start_rendering_time_);
+        start_rendering_time_ = base::TimeTicks();
+      }
     }
   }