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/system_wrappers/source/clock.cc b/webrtc/system_wrappers/source/clock.cc
index 95df256..926b95a 100644
--- a/webrtc/system_wrappers/source/clock.cc
+++ b/webrtc/system_wrappers/source/clock.cc
@@ -179,7 +179,7 @@
   }
 
   // mutable as time-accessing functions are const.
-  mutable rtc::CriticalSection crit_;
+  rtc::CriticalSection crit_;
   mutable DWORD last_time_ms_;
   mutable LONG num_timer_wraps_;
   const ReferencePoint ref_point_;