Add hacky way to send TransportFeedback in RtcpTransceiver

With an extra interface it will allow to add both RtpRtcp module
and RtcpTransceiver as feedback sender to PacketRouter

Though hacky, this is very similar to currently used implementation
in the RTCPSender::SendFeedbackPacket

Bug: webrtc:8239
Change-Id: I237b422ae1594dede78cb63daa4aa42b6774d6fe
Reviewed-on: https://webrtc-review.googlesource.com/32680
Commit-Queue: Danil Chapovalov <danilchap@webrtc.org>
Reviewed-by: Niels Moller <nisse@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#21274}
diff --git a/modules/rtp_rtcp/source/rtcp_transceiver_impl.h b/modules/rtp_rtcp/source/rtcp_transceiver_impl.h
index 83e6087..35fee75 100644
--- a/modules/rtp_rtcp/source/rtcp_transceiver_impl.h
+++ b/modules/rtp_rtcp/source/rtcp_transceiver_impl.h
@@ -49,6 +49,9 @@
 
   void SetRemb(int64_t bitrate_bps, std::vector<uint32_t> ssrcs);
   void UnsetRemb();
+  // Temporary helpers to send pre-built TransportFeedback rtcp packet.
+  uint32_t sender_ssrc() const { return config_.feedback_ssrc; }
+  void SendRawPacket(rtc::ArrayView<const uint8_t> packet);
 
   void SendNack(uint32_t ssrc, std::vector<uint16_t> sequence_numbers);