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/thread_unittest.cc b/webrtc/base/thread_unittest.cc
index 7ed4326..240ffc4 100644
--- a/webrtc/base/thread_unittest.cc
+++ b/webrtc/base/thread_unittest.cc
@@ -164,7 +164,7 @@
   }
 
  private:
-  mutable CriticalSection cs_;
+  CriticalSection cs_;
   bool flag_;
 };