[Cast] Aggressively send sender reports until first receiver report is received.

media/cast/rtcp/rtcp.h

Index: media/cast/rtcp/rtcp.h
diff --git a/media/cast/rtcp/rtcp.h b/media/cast/rtcp/rtcp.h
index efcbdf4c590a0e9516d6dd4ed2c47a720f3947e6..9d152f0e78e1465f17777e20250770a6bdb469cb 100644
--- a/media/cast/rtcp/rtcp.h
+++ b/media/cast/rtcp/rtcp.h
@@ -98,6 +98,7 @@ class Rtcp {
            base::TimeDelta* avg_rtt,
            base::TimeDelta* min_rtt,
            base::TimeDelta* max_rtt) const;
+  bool is_rtt_available() const { return number_of_rtt_in_avg_ > 0; }
   bool RtpTimestampInSenderTime(int frequency,
                                 uint32 rtp_timestamp,
                                 base::TimeTicks* rtp_timestamp_in_ticks) const;