Reset NACK/ACK list when parsing a new RTCP packet.

The NACK/ACK frames/packets list need to be reset before parsing each
RTCP packet as the parser is shared by all RTCP packets on sender side.
This caused the performance regression and potentially video freezing
as well.

BUG=600527

Review URL: https://codereview.chromium.org/1853253003

Cr-Commit-Position: refs/heads/master@{#385046}

media/cast/net/rtcp/rtcp_utility.cc

@@ -322,6 +322,8 @@ 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();
+  cast_message_.received_later_frames.clear();
   for (size_t i = 0; i < number_of_lost_fields; i++) {
     uint8_t frame_id;
     uint16_t packet_id;