Wire up rtcp xr target bitrate on receive side.

webrtc/modules/rtp_rtcp/include/rtp_rtcp.h

Index: webrtc/modules/rtp_rtcp/include/rtp_rtcp.h
diff --git a/webrtc/modules/rtp_rtcp/include/rtp_rtcp.h b/webrtc/modules/rtp_rtcp/include/rtp_rtcp.h
index b0f50755ddc93085ae94381afe4790308c1842a2..84df0118982228d6cc0f88419888daf4aadc520b 100644
--- a/webrtc/modules/rtp_rtcp/include/rtp_rtcp.h
+++ b/webrtc/modules/rtp_rtcp/include/rtp_rtcp.h
@@ -34,6 +34,7 @@ class RemoteBitrateEstimator;
 class RtcEventLog;
 class RtpReceiver;
 class Transport;
+class VideoBitrateAllocationObserver;
 
 RTPExtensionType StringToRtpExtensionType(const std::string& extension);
 
@@ -68,6 +69,7 @@ class RtpRtcp : public Module {
     RtcpBandwidthObserver* bandwidth_callback = nullptr;
 
     TransportFeedbackObserver* transport_feedback_callback = nullptr;
+    VideoBitrateAllocationObserver* bitrate_allocation_observer = nullptr;
     RtcpRttStats* rtt_stats = nullptr;
     RtcpPacketTypeCounterObserver* rtcp_packet_type_counter_observer = nullptr;