Pass a TimeTicks along video capture pipeline to represent capture time

content/public/renderer/media_stream_video_sink.h

Index: content/public/renderer/media_stream_video_sink.h
diff --git a/content/public/renderer/media_stream_video_sink.h b/content/public/renderer/media_stream_video_sink.h
index 663f3a86f072d4ca2907877ae60ef9949dda47e6..667ad3619432287ab1dfac1ee53a275b396ad08d 100644
--- a/content/public/renderer/media_stream_video_sink.h
+++ b/content/public/renderer/media_stream_video_sink.h
@@ -8,6 +8,7 @@
 #include "base/callback.h"
 #include "base/compiler_specific.h"
 #include "base/memory/ref_counted.h"
+#include "base/time/time.h"
 #include "content/common/content_export.h"
 #include "content/public/renderer/media_stream_sink.h"
 
@@ -24,7 +25,8 @@ namespace content {
 
 typedef base::Callback<
   void(const scoped_refptr<media::VideoFrame>&,
-       const media::VideoCaptureFormat&)>
+       const media::VideoCaptureFormat&,
+       const base::TimeTicks& start_ticks)>

[Ami GONE FROM CHROMIUM, 2014/05/19 19:01:46]

doco this is the stream start time.

[miu, 2014/05/19 21:59:37]

Throughout this patch: Why must we pass this value with every frame deliver
callback?  It's redundant and wasteful.  Instead, we should have a separate
callback that is only invoked once, just before the first frame, to provide the
start_time.

This would also solve a second problem throughout this patch: There are a lot of
places where you just shove a "null" TimeTicks in when invoking the delivery
callback.  With my suggestion, those implementations that do not support
providing a start_time, would simply never call the
"ProvideStartTimeTicksCallback."

I know this is extra plumbing work, but we should strive to maintain clean,
simple interfaces (which helps us write clean, simple implementations).  We've
been asking this of the WebRTC folks as they clean-up the code in
chrome/renderer/media, so we should be good citizens and do the same.

     VideoSinkDeliverFrameCB;

[Ami GONE FROM CHROMIUM, 2014/05/19 22:14:16]

Is it easy to see why VideoSinkDeliverFrameCB and VideoCaptureDeliverFrameCB
should not be the same typedef?

[Alpha Left Google, 2014/05/31 00:34:50]

I have moved the VideoCaptureDeliverFrameCB to this file to eliminate the
duplication.

 
 // MediaStreamVideoSink is an interface used for receiving video frames from a