Make FlexFEC packets paceable through RTPSender.

webrtc/modules/rtp_rtcp/source/rtp_sender_video.cc

Index: webrtc/modules/rtp_rtcp/source/rtp_sender_video.cc
diff --git a/webrtc/modules/rtp_rtcp/source/rtp_sender_video.cc b/webrtc/modules/rtp_rtcp/source/rtp_sender_video.cc
index 4a007c53eb476e798785e9b6eb4ef4566bbe8815..f772708aa0c2a60f069b31f60ed95162742bfe8a 100644
--- a/webrtc/modules/rtp_rtcp/source/rtp_sender_video.cc
+++ b/webrtc/modules/rtp_rtcp/source/rtp_sender_video.cc
@@ -275,6 +275,13 @@ void RTPSenderVideo::SetFecParameters(const FecProtectionParams& delta_params,
   key_fec_params_ = key_params;
 }
 
+rtc::Optional<uint32_t> RTPSenderVideo::FlexfecSsrc() const {
+  if (flexfec_sender_) {
+    return rtc::Optional<uint32_t>(flexfec_sender_->Ssrc());
+  }
+  return rtc::Optional<uint32_t>();  // No value.
+}
+
 bool RTPSenderVideo::SendVideo(RtpVideoCodecTypes video_type,
                                FrameType frame_type,
                                int8_t payload_type,