Break out pacer thread from CongestionController to increase testability.

webrtc/modules/congestion_controller/include/congestion_controller.h

 /*
  *  Copyright (c) 2012 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 @@
 
   // Implements Module.
   int64_t TimeUntilNextProcess() override;
   int32_t Process() override;
 
  private:
   Clock* const clock_;
   rtc::ThreadChecker config_thread_checker_;
   const rtc::scoped_ptr<PacedSender> pacer_;
   const rtc::scoped_ptr<RemoteBitrateEstimator> remote_bitrate_estimator_;
-  const rtc::scoped_ptr<ProcessThread> pacer_thread_;
   const rtc::scoped_ptr<BitrateController> bitrate_controller_;
   PacketRouter packet_router_;
   RemoteEstimatorProxy remote_estimator_proxy_;
   TransportFeedbackAdapter transport_feedback_adapter_;
   int min_bitrate_bps_;
 
   RTC_DISALLOW_IMPLICIT_CONSTRUCTORS(CongestionController);
 };
 
 }  // namespace webrtc
 
 #endif  // WEBRTC_MODULES_CONGESTION_CONTROLLER_INCLUDE_CONGESTION_CONTROLLER_H_