Simplify RtpVideoStreamReceiver::NotifyReceiverOfFecPacket.

This also has the benefit of deleting one unneeded call to
RTPPayloadRegistry::last_received_media_payload_type.

To make this work, also extend NackModule with a OnReceivedPacket
method taking only the sequence number and the is_keyframe flag,
rather than a complete VCMPacket.

Bug: webrtc:8995
Change-Id: Ice379581166e7b1609ec719e944a5a543d69acc1
Reviewed-on: https://webrtc-review.googlesource.com/64120
Reviewed-by: Erik Språng <sprang@webrtc.org>
Reviewed-by: Rasmus Brandt <brandtr@webrtc.org>
Commit-Queue: Niels Moller <nisse@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#22584}

modules/video_coding/nack_module.h

@@ -32,6 +32,7 @@ class NackModule : public Module {
              NackSender* nack_sender,
              KeyFrameRequestSender* keyframe_request_sender);
 
+  int OnReceivedPacket(uint16_t seq_num, bool is_keyframe);
   int OnReceivedPacket(const VCMPacket& packet);
   void ClearUpTo(uint16_t seq_num);
   void UpdateRtt(int64_t rtt_ms);