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

media/cast/cast_defines.h

Index: media/cast/cast_defines.h
diff --git a/media/cast/cast_defines.h b/media/cast/cast_defines.h
index faafddb8850e309b403b25530a58fa44164da2ba..c1fcea13a089c229ea97a4426948d8cc628ed2ef 100644
--- a/media/cast/cast_defines.h
+++ b/media/cast/cast_defines.h
@@ -195,6 +195,11 @@ inline base::TimeDelta RtpDeltaToTimeDelta(int64 rtp_delta, int rtp_timebase) {
   return rtp_delta * base::TimeDelta::FromSeconds(1) / rtp_timebase;
 }
 
+inline int64 TimeDeltaToRtpDelta(base::TimeDelta delta, int rtp_timebase) {
+  DCHECK_GT(rtp_timebase, 0);
+  return delta * rtp_timebase / base::TimeDelta::FromSeconds(1);
+}
+
 }  // namespace cast
 }  // namespace media