Wire up FlexfecSender in RTPSenderVideo.

webrtc/modules/rtp_rtcp/source/flexfec_sender.cc

 /*
  *  Copyright (c) 2016 The WebRTC project authors. All Rights Reserved.
  *
  *  Use of this source code is governed by a BSD-style license
  *  that can be found in the LICENSE file in the root of the source
  *  tree. An additional intellectual property rights grant can be found
  *  in the file PATENTS.  All contributing project authors may
  *  be found in the AUTHORS file in the root of the source tree.
  */
 
@@ skipping 65 matching lines @@
                                          extension.id);
     } else if (extension.uri == RtpExtension::kTimestampOffsetUri) {
       rtp_header_extension_map_.Register(kRtpExtensionTransmissionTimeOffset,
                                          extension.id);
     } else {
       LOG(LS_WARNING) << "RTP header extension with id: " << extension.id
                       << ", uri: " << extension.uri
                       << ", is unsupported by FlexfecSender.";
     }
   }
+
+  // Calculate RTP header length.
+  RtpPacketToSend rtp_header(&rtp_header_extension_map_);
+  rtp_header.ReserveExtension<AbsoluteSendTime>();
+  rtp_header.ReserveExtension<TransmissionOffset>();
+  rtp_header.ReserveExtension<TransportSequenceNumber>();
+  rtp_header_length_ = rtp_header.headers_size();

[danilchap, 2016/11/09 15:58:46]

that should be simpler:
rtp_extra_header_length_ = rtp_header_extension_map_.GetTotalLengthInBytes();
(since you do not use csrcs in the flexfec, the only header size over base 12
bytes are extensions)

[brandtr, 2016/11/10 09:32:01]

Nice, done.

 }
 
 FlexfecSender::~FlexfecSender() = default;
 
 // We are reusing the implementation from UlpfecGenerator for SetFecParameters,
 // AddRtpPacketAndGenerateFec, and FecAvailable.
 void FlexfecSender::SetFecParameters(const FecProtectionParams& params) {
   RTC_DCHECK_CALLED_SEQUENTIALLY(&sequence_checker_);
   ulpfec_generator_.SetFecParameters(params);
 }
@@ skipping 60 matching lines @@
   }
 
   return fec_packets_to_send;
 }
 
 size_t FlexfecSender::MaxPacketOverhead() const {
   return kFlexfecMaxHeaderSize;
 }
 
 }  // namespace webrtc