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

webrtc/api/statscollector.h

Index: webrtc/api/statscollector.h
diff --git a/webrtc/api/statscollector.h b/webrtc/api/statscollector.h
index 37572799fe68fcb33bc0a4bc90458604e1db318f..c17f909ef2a2f1023ab0b3714fa03e7ce69fa086 100644
--- a/webrtc/api/statscollector.h
+++ b/webrtc/api/statscollector.h
@@ -22,6 +22,7 @@
 #include "webrtc/api/peerconnectioninterface.h"
 #include "webrtc/api/statstypes.h"
 #include "webrtc/api/webrtcsession.h"
+#include "webrtc/base/thread_checker.h"
 
 namespace webrtc {
 
@@ -132,6 +133,8 @@ class StatsCollector {
   // Helper method to update the timestamp of track records.
   void UpdateTrackReports();
 
+  rtc::ThreadChecker signal_thread_checker_;
+
   // A collection for all of our stats reports.
   StatsCollection reports_;
   TrackIdMap track_ids_;