Translate loss notifications and pass to encoder

Translate LossNotification RTCP messages (sequence number to
timestamp and additional information), then send the translted
message onwards to the encoder.

Bug: webrtc:10501
Change-Id: If2fd943f75c36cf813a83120318d8eefc8c595d2
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/131950
Commit-Queue: Elad Alon <eladalon@webrtc.org>
Reviewed-by: Erik Språng <sprang@webrtc.org>
Reviewed-by: Niels Moller <nisse@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#27545}

modules/video_coding/loss_notification_controller.cc

@@ -90,6 +90,10 @@ void LossNotificationController::OnReceivedPacket(const VCMPacket& packet) {
     const bool key_frame = intra_frame;
     if (key_frame) {
       // Subsequent frames may not rely on frames before the key frame.
+      // Note that upon receiving a key frame, we do not issue a loss
+      // notification on RTP sequence number gap, unless that gap spanned
+      // the key frame itself. This is because any loss which occurred before
+      // the key frame is no longer relevant.
       decodable_unwrapped_frame_ids_.clear();
       current_frame_potentially_decodable_ = true;
     } else {