Break out pacer thread from CongestionController to increase testability.

webrtc/modules/remote_bitrate_estimator/remote_estimator_proxy.cc

Index: webrtc/modules/remote_bitrate_estimator/remote_estimator_proxy.cc
diff --git a/webrtc/modules/remote_bitrate_estimator/remote_estimator_proxy.cc b/webrtc/modules/remote_bitrate_estimator/remote_estimator_proxy.cc
index fb5d9d03c6e9eb8f38b8add5e8196cdacf00b05e..ee17ad78db8b6b5c4e8665cf467b255554e5983c 100644
--- a/webrtc/modules/remote_bitrate_estimator/remote_estimator_proxy.cc
+++ b/webrtc/modules/remote_bitrate_estimator/remote_estimator_proxy.cc
@@ -72,8 +72,6 @@ int64_t RemoteEstimatorProxy::TimeUntilNextProcess() {
 }
 
 int32_t RemoteEstimatorProxy::Process() {
-  // TODO(sprang): Perhaps we need a dedicated thread here instead?
-
   if (TimeUntilNextProcess() > 0)
     return 0;
   last_process_time_ms_ = clock_->TimeInMilliseconds();