Calls OnPacketsAcknowledged on RtpRtcp instead of RTPSender directly.

This prepares for splitting RtpSenderEgress out of RTPSender.
For context, see:
https://webrtc-review.googlesource.com/c/src/+/158020

Bug: webrtc:11036
Change-Id: I6d385ba255ce23f4c6685a3737eeb243ce2ec6ff
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/158201
Reviewed-by: Sebastian Jansson <srte@webrtc.org>
Commit-Queue: Erik Språng <sprang@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#29601}

modules/rtp_rtcp/include/rtp_rtcp.h

@@ -276,6 +276,9 @@ class RtpRtcp : public Module, public RtcpFeedbackSenderInterface {
   virtual bool TrySendPacket(RtpPacketToSend* packet,
                              const PacedPacketInfo& pacing_info) = 0;
 
+  virtual void OnPacketsAcknowledged(
+      rtc::ArrayView<const uint16_t> sequence_numbers) = 0;
+
   virtual std::vector<std::unique_ptr<RtpPacketToSend>> GeneratePadding(
       size_t target_size_bytes) = 0;