Remove mutable from rtc::CriticalSection members.

webrtc/base/thread_checker_impl.h

 /*
  *  Copyright (c) 2014 The WebRTC project authors. All Rights Reserved.
  *
  *  Use of this source code is governed by a BSD-style license
  *  that can be found in the LICENSE file in the root of the source
  *  tree. An additional intellectual property rights grant can be found
  *  in the file PATENTS.  All contributing project authors may
  *  be found in the AUTHORS file in the root of the source tree.
  */
 
@@ skipping 19 matching lines @@
   ~ThreadCheckerImpl();
 
   bool CalledOnValidThread() const;
 
   // Changes the thread that is checked for in CalledOnValidThread.  This may
   // be useful when an object may be created on one thread and then used
   // exclusively on another thread.
   void DetachFromThread();
 
  private:
-  mutable CriticalSection lock_;
+  CriticalSection lock_;
   // This is mutable so that CalledOnValidThread can set it.
   // It's guarded by |lock_|.
   mutable PlatformThreadRef valid_thread_;
 };
 
 }  // namespace rtc
 
 #endif  // WEBRTC_BASE_THREAD_CHECKER_IMPL_H_