Preliminary change for new rtc rendering algorithm

media/base/video_frame.h

Index: media/base/video_frame.h
diff --git a/media/base/video_frame.h b/media/base/video_frame.h
index bc92b9a0fcaee3c460ee2cf493b2629cae7c2fc7..b03c9c6120d13b7cbd90bd25ad7797f5e577c5b9 100644
--- a/media/base/video_frame.h
+++ b/media/base/video_frame.h
@@ -376,6 +376,10 @@ class MEDIA_EXPORT VideoFrame : public base::RefCountedThreadSafe<VideoFrame> {
   // This method is thread safe. Both blink and compositor threads can call it.
   void UpdateReleaseSyncPoint(SyncPointClient* client);
 
+  // Record the ideal time instant to render this frame.

[DaleCurtis, 2015/07/30 05:55:24]

VideoFrame is already in a crufty situation so we want to avoid adding any more
one off fields like this. Can you deliver the render time through another
mechanism? Either by set_timestamp(render_time - base::TimeTicks()) or via a new
VideoFrameMetadata field?

[qiangchen, 2015/07/30 17:40:26]

I'll consider using VideoFrameMetadata. Should I also remove the methods, and
let the caller to retrieve information from metadata?

P.S.

Now the timestamp of media::VideoFrame is essentially the RTP timestamp/90000. 
While the render_time calculated in Jitter Buffer is derived by an equation
fitting not from a static formula, which is necessary and powerful as it can
kill the clock rate inconsistency. 

Thus we should use render_time as the guidance of frame selection. But I'm not
sure if we simply set timestamp to be render_time, the loss of the raw timestamp
would affect something else.

[DaleCurtis, 2015/07/30 17:52:33]

Before you go the metadata approach, I would see if you can just overwrite the
existing timestamp() field at this point; it shouldn't be too hard to
investigate the few call sites within WebRTC that use timestamp().

Definitely we don't want a new render_time() and set_render_time() on VideoFrame
though; either you'll need to add a new VideoFrameMetadata field or stuff the
value into the timestamp field (most ideal).

+  base::TimeTicks render_time();
+  void set_render_time(base::TimeTicks render_time);
+
  private:
   friend class base::RefCountedThreadSafe<VideoFrame>;
 
@@ -478,6 +482,8 @@ class MEDIA_EXPORT VideoFrame : public base::RefCountedThreadSafe<VideoFrame> {
 
   VideoFrameMetadata metadata_;
 
+  base::TimeTicks render_time_;
+
   DISALLOW_IMPLICIT_CONSTRUCTORS(VideoFrame);
 };