Move congestion controller processing to the pacer thread.

webrtc/modules/pacing/alr_detector.cc

Index: webrtc/modules/pacing/alr_detector.cc
diff --git a/webrtc/modules/pacing/alr_detector.cc b/webrtc/modules/pacing/alr_detector.cc
index b31d4482bba37896bb7b0709d7e796d6e8c6eb53..9a917983be20c53169100e7e1001eeeafd8b9997 100644
--- a/webrtc/modules/pacing/alr_detector.cc
+++ b/webrtc/modules/pacing/alr_detector.cc
@@ -36,6 +36,10 @@ AlrDetector::AlrDetector()
 AlrDetector::~AlrDetector() {}
 
 void AlrDetector::OnBytesSent(size_t bytes_sent, int64_t now_ms) {
+  // TODO(nisse): It's unclear what guarantees there are that this
+  // function isn't called before SetEstimatedBitrate. Document that
+  // guarantee, if it exists. Or else, remove the DCHECK and tolerate
+  // that current estimate is zero, e.g., by just returning false.
   RTC_DCHECK(estimated_bitrate_bps_);
 
   rate_.Update(bytes_sent, now_ms);