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

media/blink/video_frame_compositor.h

Index: media/blink/video_frame_compositor.h
diff --git a/media/blink/video_frame_compositor.h b/media/blink/video_frame_compositor.h
index 31857c9ee0bd543430d302c5e74e227c9ee39edc..5048c5f5b19429f1e350a9d4c3874c9fdc82cd91 100644
--- a/media/blink/video_frame_compositor.h
+++ b/media/blink/video_frame_compositor.h
@@ -103,6 +103,11 @@ class MEDIA_BLINK_EXPORT VideoFrameCompositor
   // PaintSingleFrame() is not also called while stopped.)
   base::TimeDelta GetCurrentFrameTimestamp() const;
 
+  // Called when the video becomes visible with the timestamp.

[DaleCurtis, 2017/01/05 22:38:43]

visible is a bit confusing in this context; instead you should write this in
terms of a background to foreground transition. Maybe something like
SetForegroundTime(). Per our typical lexicon Timestamp is only used for
TimeDelta's, so this should just be XXXTime().

[whywhat, 2017/01/05 23:15:44]

s/Timestamp/Time sgtm

Re: s/shown/foreground:
1) there's another "background" term used here w.r.t. the background thread
being used for rendering IIUC -- I already used |is_background_rendering_| in
some patchset here incorrectly because of the term reuse.
2) I hope in the future the term shown will suit these transitions better as
we'll reuse the media remoting work that detects when the media elements is in
the viewport and not (so visible/shown == tab is in foreground _and_ the element
is in the viewport)

Therefore I used Shown/Hidden to be consistent with OnHidden/OnShown in WMPI and
not to be confused with background render in VFC.

[DaleCurtis, 2017/01/05 23:21:46]

Background rendering and SetForegroundTime() would be fine, they are both
effectively triggered by the inverse of the other.

+  // Used to record the time it takes to process the first frame after that.
+  // Must be called on the compositor thread.
+  void SetShownTimestamp(base::TimeTicks when);
+
   void set_tick_clock_for_testing(std::unique_ptr<base::TickClock> tick_clock) {
     tick_clock_ = std::move(tick_clock);
   }
@@ -157,6 +162,7 @@ class MEDIA_BLINK_EXPORT VideoFrameCompositor
   bool new_background_frame_;
   base::TimeDelta last_interval_;
   base::TimeTicks last_background_render_;
+  base::TimeTicks last_shown_;
 
   // These values are set on the compositor thread, but also read on the media
   // thread when the VFC is stopped.