Add Cast PLI support on sender side.

media/cast/net/rtcp/rtcp_builder.cc

Index: media/cast/net/rtcp/rtcp_builder.cc
diff --git a/media/cast/net/rtcp/rtcp_builder.cc b/media/cast/net/rtcp/rtcp_builder.cc
index cd7652307837fa64314ba4607b7c62d34c0954e6..0d91f243e1f9e80a2234b9826bc4dee08dc1fb8f 100644
--- a/media/cast/net/rtcp/rtcp_builder.cc
+++ b/media/cast/net/rtcp/rtcp_builder.cc
@@ -165,6 +165,7 @@ PacketRef RtcpBuilder::BuildRtcpFromReceiver(
     const RtcpReportBlock* report_block,
     const RtcpReceiverReferenceTimeReport* rrtr,
     const RtcpCastMessage* cast_message,
+    const RtcpPliMessage* pli_message,
     const ReceiverRtcpEventSubscriber::RtcpEvents* rtcp_events,
     base::TimeDelta target_delay) {
   Start();
@@ -175,6 +176,8 @@ PacketRef RtcpBuilder::BuildRtcpFromReceiver(
     AddRrtr(rrtr);
   if (cast_message)
     AddCast(cast_message, target_delay);
+  if (pli_message)
+    AddPli(pli_message);
   if (rtcp_events)
     AddReceiverLog(*rtcp_events);
 
@@ -227,6 +230,12 @@ void RtcpBuilder::AddRrtr(const RtcpReceiverReferenceTimeReport* rrtr) {
   writer_.WriteU32(rrtr->ntp_fraction);
 }
 
+void RtcpBuilder::AddPli(const RtcpPliMessage* pli_message) {
+  AddRtcpHeader(kPacketTypePayloadSpecific, 1);
+  writer_.WriteU32(ssrc_);
+  writer_.WriteU32(pli_message->sender_ssrc);
+}
+
 void RtcpBuilder::AddCast(const RtcpCastMessage* cast,
                           base::TimeDelta target_delay) {
   // See RTC 4585 Section 6.4 for application specific feedback messages.