[Cast] In Audio/VideoSender, drop frames when too-long a duration is in-flight.

media/cast/sender/rtp_timestamp_helper.h

Index: media/cast/sender/rtp_timestamp_helper.h
diff --git a/media/cast/sender/rtp_timestamp_helper.h b/media/cast/sender/rtp_timestamp_helper.h
index 8f56681dac3651579884f8ce619a61718677c21a..2146e22d5412200c72352ff97950e13fbc4748e9 100644
--- a/media/cast/sender/rtp_timestamp_helper.h
+++ b/media/cast/sender/rtp_timestamp_helper.h
@@ -17,11 +17,13 @@ class RtpTimestampHelper {
   explicit RtpTimestampHelper(int frequency);
   ~RtpTimestampHelper();
 
-  // Compute a RTP timestamp using current time, last encoded time and
-  // last encoded RTP timestamp.
-  // Return true if |rtp_timestamp| is computed.
-  bool GetCurrentTimeAsRtpTimestamp(const base::TimeTicks& now,
-                                    uint32* rtp_timestamp) const;
+  int frequency() const { return frequency_; }
+
+  // Compute a RTP timestamp for |t|, based on the last stored capture time and
+  // its corresponding RTP timestamp.  Return true if |rtp_timestamp| is
+  // computed.
+  bool EstimateRtpTimestamp(const base::TimeTicks& t,

[hubbe, 2014/08/26 17:49:56]

Why is it an estimate?
Does it return "approximately" the right timestamp?

[miu, 2014/08/26 19:57:19]

Yes.  It's an estimate because, ultimately, it's the encoders that decide what
the RTP timestamps for each frame are going to be.  For example, in the case of
audio, the record_time will never be precise to a 10ms boundary; and so the
translation will be to an |rtp_timestamp| value that is near, but never exact to
one emitted downstream by the encoder.  In other words, it's an approximation.

[hubbe, 2014/08/26 20:54:21]

If we do this with timeticks/deltas instead, do we even need this estimate? If
the answer is yes, please update the comment to provide the same information as
your reply above.

[miu, 2014/08/26 23:27:16]

Done.  This method is also used when generating SR's, so I explained the
"estimate" via code comments.

+                            uint32* rtp_timestamp) const;
 
   // Store the capture time and the corresponding RTP timestamp for the
   // last encoded frame.