Re-land (rebased for M50): Reset NACK/ACK list when parsing a new RTCP packet.

media/cast/net/rtcp/rtcp_utility.cc

Index: media/cast/net/rtcp/rtcp_utility.cc
diff --git a/media/cast/net/rtcp/rtcp_utility.cc b/media/cast/net/rtcp/rtcp_utility.cc
index 74af3a04fb45ab8b3998c185d80cafdcda4e49a4..887de3a4e6ac4d918b39697a2f9c7c0fc05481c6 100644
--- a/media/cast/net/rtcp/rtcp_utility.cc
+++ b/media/cast/net/rtcp/rtcp_utility.cc
@@ -291,6 +291,7 @@ bool RtcpParser::ParseFeedbackCommon(base::BigEndianReader* reader,
   // Please note, this frame_id is still only 8-bit!
   cast_message_.ack_frame_id = last_frame_id;
 
+  cast_message_.missing_frames_and_packets.clear();
   for (size_t i = 0; i < number_of_lost_fields; i++) {
     uint8_t frame_id;
     uint16_t packet_id;