Break out pacer thread from CongestionController to increase testability.

webrtc/modules/remote_bitrate_estimator/remote_estimator_proxy.cc

 /*
  *  Copyright (c) 2015 The WebRTC project authors. All Rights Reserved.
  *
  *  Use of this source code is governed by a BSD-style license
  *  that can be found in the LICENSE file in the root of the source
  *  tree. An additional intellectual property rights grant can be found
  *  in the file PATENTS.  All contributing project authors may
  *  be found in the AUTHORS file in the root of the source tree.
  */
 
@@ skipping 54 matching lines @@
   int64_t now = clock_->TimeInMilliseconds();
   int64_t time_until_next = 0;
   if (last_process_time_ms_ != -1 &&
       now - last_process_time_ms_ < kDefaultProcessIntervalMs) {
     time_until_next = (last_process_time_ms_ + kDefaultProcessIntervalMs - now);
   }
   return time_until_next;
 }
 
 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();
 
   bool more_to_build = true;
   while (more_to_build) {
     rtcp::TransportFeedback feedback_packet;
     if (BuildFeedbackPacket(&feedback_packet)) {
       RTC_DCHECK(packet_router_ != nullptr);
       packet_router_->SendFeedback(&feedback_packet);
@@ skipping 62 matching lines @@
     // they need to be re-sent after a reordering. Removal will be handled
     // by OnPacketArrival once packets are too old.
   }
   if (it == packet_arrival_times_.end())
     window_start_seq_ = -1;
 
   return true;
 }
 
 }  // namespace webrtc