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/base/physicalsocketserver.h b/webrtc/base/physicalsocketserver.h
index ae1f10f..cbe6580 100644
--- a/webrtc/base/physicalsocketserver.h
+++ b/webrtc/base/physicalsocketserver.h
@@ -172,7 +172,7 @@
   SOCKET s_;
   uint8_t enabled_events_;
   bool udp_;
-  mutable CriticalSection crit_;
+  CriticalSection crit_;
   int error_ GUARDED_BY(crit_);
   ConnState state_;
   AsyncResolver* resolver_;