Consolidate FEC book-keeping

Number of received FEC bytes is used for the
WebRTC.Video.FecBitrateReceivedInKbps UMA histogram. Before this cl,
that value is based on a FEC packet counter updated by
ReceiveStatistics::FecPacketReceived. This cl deletes that method, and
instead adds a byte count to the FecPacketCounter struct, which is
maintained by the UlpFecReceiver and used for other FEC-related stats.

Bug: webrtc:10917
Change-Id: I24bd494b6909a2fe109d28e2b71ca8f413d05911
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/150533
Reviewed-by: Åsa Persson <asapersson@webrtc.org>
Commit-Queue: Niels Moller <nisse@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#28976}

modules/rtp_rtcp/source/ulpfec_receiver_impl.cc

@@ -122,6 +122,7 @@ int32_t UlpfecReceiverImpl::AddReceivedRedPacket(
   }
 
   ++packet_counter_.num_packets;
+  packet_counter_.num_bytes += packet_length;
   if (packet_counter_.first_packet_time_ms == -1) {
     packet_counter_.first_packet_time_ms = rtc::TimeMillis();
   }