Make FlexFEC packets paceable through RTPSender.

webrtc/modules/rtp_rtcp/source/rtp_sender_video.h

Index: webrtc/modules/rtp_rtcp/source/rtp_sender_video.h
diff --git a/webrtc/modules/rtp_rtcp/source/rtp_sender_video.h b/webrtc/modules/rtp_rtcp/source/rtp_sender_video.h
index da016b767e3748e986c47ed32334dce5051cc38f..853507dd752df61cf41c5aa79ba1097e36c7796c 100644
--- a/webrtc/modules/rtp_rtcp/source/rtp_sender_video.h
+++ b/webrtc/modules/rtp_rtcp/source/rtp_sender_video.h
@@ -17,6 +17,7 @@
 
 #include "webrtc/base/criticalsection.h"
 #include "webrtc/base/onetimeevent.h"
+#include "webrtc/base/optional.h"
 #include "webrtc/base/rate_statistics.h"
 #include "webrtc/base/sequenced_task_checker.h"
 #include "webrtc/base/thread_annotations.h"
@@ -66,9 +67,13 @@ class RTPSenderVideo {
   void SetUlpfecConfig(int red_payload_type, int ulpfec_payload_type);
   void GetUlpfecConfig(int* red_payload_type, int* ulpfec_payload_type) const;
 
+  // FlexFEC/ULPFEC.
   void SetFecParameters(const FecProtectionParams& delta_params,
                         const FecProtectionParams& key_params);
 
+  // FlexFEC.
+  rtc::Optional<uint32_t> FlexfecSsrc() const;
+
   uint32_t VideoBitrateSent() const;
   uint32_t FecOverheadRate() const;