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.h

@@ -44,6 +44,8 @@ class RtcpTransceiver {
   // Stops sending REMB in following compound packets.
   void UnsetRemb();
 
+  void RequestKeyFrame(std::vector<uint32_t> ssrcs);
+
  private:
   rtc::TaskQueue* const task_queue_;
   std::unique_ptr<RtcpTransceiverImpl> rtcp_transceiver_;