Remove mutable from rtc::CriticalSection members.

rtc::CriticalSection is now lockable from const methods and no longer
need to remain mutable.

BUG=
R=tommi@webrtc.org

Review URL: https://codereview.webrtc.org/1613643004

Cr-Commit-Position: refs/heads/master@{#11367}
diff --git a/webrtc/call/congestion_controller.h b/webrtc/call/congestion_controller.h
index b77c46f..5f5e590 100644
--- a/webrtc/call/congestion_controller.h
+++ b/webrtc/call/congestion_controller.h
@@ -74,7 +74,7 @@
   rtc::scoped_ptr<RemoteBitrateEstimator> remote_bitrate_estimator_;
   rtc::scoped_ptr<RemoteEstimatorProxy> remote_estimator_proxy_;
 
-  mutable rtc::CriticalSection encoder_crit_;
+  rtc::CriticalSection encoder_crit_;
   std::vector<ViEEncoder*> encoders_ GUARDED_BY(encoder_crit_);
 
   // Registered at construct time and assumed to outlive this class.