Add Cast PLI support on sender side.

media/cast/net/rtcp/test_rtcp_packet_builder.cc

Index: media/cast/net/rtcp/test_rtcp_packet_builder.cc
diff --git a/media/cast/net/rtcp/test_rtcp_packet_builder.cc b/media/cast/net/rtcp/test_rtcp_packet_builder.cc
index 67cd6edf1a09b9b2e6a4c9671589c68198825d7e..130c5d22364524bf96ce76fd0c55e7e88c804f98 100644
--- a/media/cast/net/rtcp/test_rtcp_packet_builder.cc
+++ b/media/cast/net/rtcp/test_rtcp_packet_builder.cc
@@ -183,6 +183,12 @@ void TestRtcpPacketBuilder::AddCst2(
   }
 }
 
+void TestRtcpPacketBuilder::AddPli(uint32_t sender_ssrc, uint32_t media_ssrc) {
+  AddRtcpHeader(206, 1);
+  big_endian_writer_.WriteU32(sender_ssrc);
+  big_endian_writer_.WriteU32(media_ssrc);
+}
+
 void TestRtcpPacketBuilder::AddReceiverLog(uint32_t sender_ssrc) {
   AddRtcpHeader(204, 2);
   big_endian_writer_.WriteU32(sender_ssrc);