Add RequestKeyFrame with Pli to RtcpTransceiver

Add support for reduced size mode.

Bug: webrtc:8239
Change-Id: I1d646f0d7848af6632c9204ce5b96ae24cfc0ad3
Reviewed-on: https://webrtc-review.googlesource.com/23681
Commit-Queue: Danil Chapovalov <danilchap@webrtc.org>
Reviewed-by: Niels Moller <nisse@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#20812}

modules/rtp_rtcp/source/rtcp_transceiver_config.h

@@ -13,6 +13,7 @@
 
 #include <string>
 
+#include "modules/rtp_rtcp/include/rtp_rtcp_defines.h"
 #include "rtc_base/task_queue.h"
 
 namespace webrtc {
@@ -51,6 +52,11 @@ struct RtcpTransceiverConfig {
   // Rtcp report block generator for outgoing receiver reports.
   ReceiveStatisticsProvider* receive_statistics = nullptr;
 
+  // Configures if sending should
+  //  enforce compound packets: https://tools.ietf.org/html/rfc4585#section-3.1
+  //  or allow reduced size packets: https://tools.ietf.org/html/rfc5506
+  // Receiving accepts both compound and reduced-size packets.
+  RtcpMode rtcp_mode = RtcpMode::kCompound;
   //
   // Tuning parameters.
   //