Preliminary change for new rtc rendering algorithm

media/base/video_frame_metadata.h

Index: media/base/video_frame_metadata.h
diff --git a/media/base/video_frame_metadata.h b/media/base/video_frame_metadata.h
index bd330f0ed3bfa598ded89b9cc9915a58e7405ca0..caf583bf77529d5ce2f45d7490eb56c39b4c347f 100644
--- a/media/base/video_frame_metadata.h
+++ b/media/base/video_frame_metadata.h
@@ -68,6 +68,16 @@ class MEDIA_EXPORT VideoFrameMetadata {
     // measurements would be used as feedback.
     RESOURCE_UTILIZATION,
 
+    // Render side: a TimeTicks that records the ideal time instant to render

[miu, 2015/08/04 04:35:08]

As this concept is confusing to a lot of media developers, can you provide more
detail?  FWIW, you could pretty much copy-and-paste the comment from here (with
minor changes):
https://code.google.com/p/chromium/codesearch#chromium/src/media/base/video_c...

[qiangchen, 2015/08/04 16:35:25]

Done.

+    // the current frame.
+    // Capture side: a TimeTicks that records the capture time.
+    REFERENCE_TIME,

[miu, 2015/08/04 04:35:08]

Please move this up (for alphabetizing).

[qiangchen, 2015/08/04 16:35:25]

Done.

+
+    // A TimeDelta measuring the time between the current frame and the first
+    // frame of the stream.
+    // TODO(qiangchen): Move VideoFrame::timestamp() to this KEY.
+    ELAPSED_TIME,

[miu, 2015/08/04 04:35:08]

Let's not add ELAPSED_TIME yet.  We can do that in a future change, if we decide
to move VideoFrame::timestamp() into the metadata.

[qiangchen, 2015/08/04 16:35:25]

Done.

+
     NUM_KEYS
   };