Remove mutable from rtc::CriticalSection members.

webrtc/system_wrappers/source/clock.cc

Index: webrtc/system_wrappers/source/clock.cc
diff --git a/webrtc/system_wrappers/source/clock.cc b/webrtc/system_wrappers/source/clock.cc
index 95df256f3cf747951f96a22d5b839b6e6be84414..926b95a3dfd67a0bd6b269c72cdbfb7ef4b407ec 100644
--- a/webrtc/system_wrappers/source/clock.cc
+++ b/webrtc/system_wrappers/source/clock.cc
@@ -179,7 +179,7 @@ class WindowsRealTimeClock : public RealTimeClock {
   }
 
   // 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_;