Forward LossNotification from RTCPReceiver to EncoderRtcpFeedback

TBR=sprang@webrtc.org

Bug: webrtc:10501
Change-Id: I09a571a65ba8515b027ee32d1f46e5cc7f699704
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/131325
Reviewed-by: Elad Alon <eladalon@webrtc.org>
Reviewed-by: Danil Chapovalov <danilchap@webrtc.org>
Commit-Queue: Elad Alon <eladalon@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#27513}

modules/rtp_rtcp/include/rtp_rtcp.h

@@ -64,9 +64,12 @@ class RtpRtcp : public Module, public RtcpFeedbackSenderInterface {
     // out on the network.
     Transport* outgoing_transport = nullptr;
 
-    // Called when the receiver request a intra frame.
+    // Called when the receiver requests an intra frame.
     RtcpIntraFrameObserver* intra_frame_callback = nullptr;
 
+    // Called when the receiver sends a loss notification.
+    RtcpLossNotificationObserver* rtcp_loss_notification_observer = nullptr;
+
     // Called when we receive a changed estimate from the receiver of out
     // stream.
     RtcpBandwidthObserver* bandwidth_callback = nullptr;