Wire up FlexfecSender in RTPSenderVideo.

webrtc/modules/rtp_rtcp/include/flexfec_sender.h

Index: webrtc/modules/rtp_rtcp/include/flexfec_sender.h
diff --git a/webrtc/modules/rtp_rtcp/include/flexfec_sender.h b/webrtc/modules/rtp_rtcp/include/flexfec_sender.h
index f50dfb6b39a14d6f22631ae9b67f6cc0bdaf120e..058e0160f654fe6d231ac195364cfdb297876aea 100644
--- a/webrtc/modules/rtp_rtcp/include/flexfec_sender.h
+++ b/webrtc/modules/rtp_rtcp/include/flexfec_sender.h
@@ -74,7 +74,7 @@ class FlexfecSender {
 
   // 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