Wire up FlexfecSender in RTPSenderVideo.

webrtc/modules/rtp_rtcp/include/flexfec_sender.h

 /*
  *  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 56 matching lines @@
   // Config.
   const int payload_type_;
   const uint32_t timestamp_offset_;
   const uint32_t ssrc_;
   const uint32_t protected_media_ssrc_;
   // Sequence number of next packet to generate.
   uint16_t seq_num_ GUARDED_BY(sequence_checker_);
 
   // Implementation.
   UlpfecGenerator ulpfec_generator_ GUARDED_BY(sequence_checker_);
-  RtpHeaderExtensionMap rtp_header_extension_map_ GUARDED_BY(sequence_checker_);
+  const RtpHeaderExtensionMap rtp_header_extension_map_;
 };
 
 }  // namespace webrtc
 
 #endif  // WEBRTC_MODULES_RTP_RTCP_INCLUDE_FLEXFEC_SENDER_H_