Late initialize MediaController, for less resource i.e. ProcessThread, usage by PeerConnection.

webrtc/api/statscollector.cc

Index: webrtc/api/statscollector.cc
diff --git a/webrtc/api/statscollector.cc b/webrtc/api/statscollector.cc
index ca1548fa0d53531a6af4ea4c2a5b69d4f2a4b8f9..7fb0c2b7b3aef4946ddd9e73546dcab22c3c2f90 100644
--- a/webrtc/api/statscollector.cc
+++ b/webrtc/api/statscollector.cc
@@ -462,6 +462,8 @@ StatsCollector::UpdateStats(PeerConnectionInterface::StatsOutputLevel level) {
   }
   stats_gathering_started_ = time_now;
 
+  // TODO(pthatcher): Merge PeerConnection and WebRtcSession so there is no
+  // pc_->session().
   if (pc_->session()) {
     // TODO(tommi): All of these hop over to the worker thread to fetch
     // information.  We could use an AsyncInvoker to run all of these and post