Move congestion controller processing to the pacer thread.

webrtc/modules/pacing/paced_sender.cc

Index: webrtc/modules/pacing/paced_sender.cc
diff --git a/webrtc/modules/pacing/paced_sender.cc b/webrtc/modules/pacing/paced_sender.cc
index a0d7eda73f26f901afbf29de7abc051034b68ae9..55a060cbc381f0987692912d1d81f50ee26e4f7f 100644
--- a/webrtc/modules/pacing/paced_sender.cc
+++ b/webrtc/modules/pacing/paced_sender.cc
@@ -450,9 +450,11 @@ void PacedSender::Process() {
         bytes_sent += SendPadding(padding_needed, probe_cluster_id);
     }
   }
-  if (is_probing && bytes_sent > 0)
-    prober_->ProbeSent(clock_->TimeInMilliseconds(), bytes_sent);
-  alr_detector_->OnBytesSent(bytes_sent, now_us / 1000);
+  if (bytes_sent > 0) {
+    if (is_probing)
+      prober_->ProbeSent(clock_->TimeInMilliseconds(), bytes_sent);
+    alr_detector_->OnBytesSent(bytes_sent, now_us / 1000);
+  }
 }
 
 bool PacedSender::SendPacket(const paced_sender::Packet& packet,