Add thread check to PacketRouter::SendFeedback, which is called on the pacer thread.

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 fcb1e4976c5cad62eb202f07ec8b20ea9a7b7899..762e3c581c3b167e7003934879044221dc36d364 100644
--- a/webrtc/modules/pacing/packet_router.cc
+++ b/webrtc/modules/pacing/packet_router.cc
@@ -106,6 +106,7 @@ uint16_t PacketRouter::AllocateSequenceNumber() {
 }
 
 bool PacketRouter::SendFeedback(rtcp::TransportFeedback* packet) {
+  RTC_DCHECK(pacer_thread_checker_.CalledOnValidThread());
   rtc::CritScope cs(&modules_crit_);
   for (auto* rtp_module : rtp_modules_) {
     packet->SetSenderSsrc(rtp_module->SSRC());