Add Cast PLI support on sender side.

media/cast/net/rtcp/rtcp_defines.cc

Index: media/cast/net/rtcp/rtcp_defines.cc
diff --git a/media/cast/net/rtcp/rtcp_defines.cc b/media/cast/net/rtcp/rtcp_defines.cc
index b4f363e30a14625b286a94eb5936f840244b57ad..c9afa7134791bb699d4b320063c206e41dade909 100644
--- a/media/cast/net/rtcp/rtcp_defines.cc
+++ b/media/cast/net/rtcp/rtcp_defines.cc
@@ -15,6 +15,10 @@ RtcpCastMessage::RtcpCastMessage()
     : media_ssrc(0), ack_frame_id(0u), target_delay_ms(0) {}
 RtcpCastMessage::~RtcpCastMessage() {}
 
+RtcpPliMessage::RtcpPliMessage(uint32_t ssrc) : sender_ssrc(ssrc) {}
+RtcpPliMessage::RtcpPliMessage() : sender_ssrc(0) {}
+RtcpPliMessage::~RtcpPliMessage() {}
+
 RtcpReceiverEventLogMessage::RtcpReceiverEventLogMessage()
     : type(UNKNOWN), packet_id(0u) {}
 RtcpReceiverEventLogMessage::~RtcpReceiverEventLogMessage() {}