Change return type of CongestionController::pacer() method.

webrtc/modules/congestion_controller/congestion_controller.cc

Index: webrtc/modules/congestion_controller/congestion_controller.cc
diff --git a/webrtc/modules/congestion_controller/congestion_controller.cc b/webrtc/modules/congestion_controller/congestion_controller.cc
index 44b2f41e86b85117b1e121ec6c56de1a028c29d4..7c492c4813743345a8b47223bf088b79a98e59c8 100644
--- a/webrtc/modules/congestion_controller/congestion_controller.cc
+++ b/webrtc/modules/congestion_controller/congestion_controller.cc
@@ -23,6 +23,7 @@
 #include "webrtc/modules/remote_bitrate_estimator/include/bwe_defines.h"
 #include "webrtc/modules/remote_bitrate_estimator/remote_bitrate_estimator_abs_send_time.h"
 #include "webrtc/modules/remote_bitrate_estimator/remote_bitrate_estimator_single_stream.h"
+#include "webrtc/modules/utility/include/process_thread.h"
 #include "webrtc/system_wrappers/include/critical_section_wrapper.h"
 
 namespace webrtc {
@@ -173,6 +174,7 @@ CongestionController::CongestionController(
     : clock_(clock),
       observer_(observer),
       packet_router_(packet_router),
+      pacer_thread_(ProcessThread::Create("PacerThread")),

[perkj_webrtc, 2017/01/02 09:18:58]

There are upstreams projects that use CongestionController but not the pacer
that does not want an extra thread. 
Why is the thread created here when its not used in this class? Can it be
created and owned by the pacer itself? 

[nisse-webrtc, 2017/01/09 13:43:46]

I can try moving it to the pacer itself. One obvious obstacle is the calls to
RegisterModule and DeRegisterModule with &remote_estimator_proxy_.

Or is it bad to have the remote_estimator_proxy and pacer share the same thread?
If they do, it makes some sense to me to let the CongestionController own it.

       pacer_(std::move(pacer)),
       remote_bitrate_estimator_(
           new WrappingBitrateEstimator(remote_bitrate_observer, clock_)),
@@ -193,9 +195,18 @@ CongestionController::CongestionController(
       network_state_(kNetworkUp) {
   transport_feedback_adapter_.InitBwe();
   transport_feedback_adapter_.SetMinBitrate(min_bitrate_bps_);
+
+  pacer_thread_->RegisterModule(pacer_.get());
+  pacer_thread_->RegisterModule(&remote_estimator_proxy_);
+  pacer_thread_->Start();
+
 }
 
-CongestionController::~CongestionController() {}
+CongestionController::~CongestionController() {
+  pacer_thread_->Stop();
+  pacer_thread_->DeRegisterModule(pacer_.get());
+  pacer_thread_->DeRegisterModule(&remote_estimator_proxy_);
+}
 
 void CongestionController::SetBweBitrates(int min_bitrate_bps,
                                           int start_bitrate_bps,