Make FlexFEC packets paceable through RTPSender.

webrtc/modules/pacing/packet_router.cc

Index: webrtc/modules/pacing/packet_router.cc
diff --git a/webrtc/modules/pacing/packet_router.cc b/webrtc/modules/pacing/packet_router.cc
index de8c2cee5a13c106501ccc4f47e1656866c2c57f..85d9ec0ab7bdec585a04fca19cce0a9d32d41821 100644
--- a/webrtc/modules/pacing/packet_router.cc
+++ b/webrtc/modules/pacing/packet_router.cc
@@ -48,7 +48,11 @@ bool PacketRouter::TimeToSendPacket(uint32_t ssrc,
   RTC_DCHECK(pacer_thread_checker_.CalledOnValidThread());
   rtc::CritScope cs(&modules_crit_);
   for (auto* rtp_module : rtp_modules_) {
-    if (rtp_module->SendingMedia() && ssrc == rtp_module->SSRC()) {
+    if (!rtp_module->SendingMedia())
+      continue;
+    uint32_t media_ssrc = rtp_module->SSRC();
+    rtc::Optional<uint32_t> flexfec_ssrc = rtp_module->FlexfecSsrc();
+    if (ssrc == media_ssrc || (flexfec_ssrc && ssrc == flexfec_ssrc)) {
       return rtp_module->TimeToSendPacket(ssrc, sequence_number,
                                           capture_timestamp, retransmission,
                                           probe_cluster_id);