Remove base::NonThreadSafe now that all users are gone!!

base/threading/non_thread_safe.h

-// Copyright (c) 2012 The Chromium Authors. All rights reserved.
-// Use of this source code is governed by a BSD-style license that can be
-// found in the LICENSE file.
-
-#ifndef BASE_THREADING_NON_THREAD_SAFE_H_
-#define BASE_THREADING_NON_THREAD_SAFE_H_
-
-// Classes deriving from NonThreadSafe may need to suppress MSVC warning 4275:
-// non dll-interface class 'Bar' used as base for dll-interface class 'Foo'.
-// There is a specific macro to do it: NON_EXPORTED_BASE(), defined in
-// compiler_specific.h
-#include "base/compiler_specific.h"
-#include "base/logging.h"
-#include "base/threading/non_thread_safe_impl.h"
-
-namespace base {
-
-// Do nothing implementation of NonThreadSafe, for release mode.
-//
-// Note: You should almost always use the NonThreadSafe class to get
-// the right version of the class for your build configuration.
-class NonThreadSafeDoNothing {
- public:
-  bool CalledOnValidThread() const {
-    return true;
-  }
-
- protected:
-  ~NonThreadSafeDoNothing() {}
-  void DetachFromThread() {}
-};
-
-// DEPRECATED! Use base::SequenceChecker (for thread-safety) or
-// base::ThreadChecker (for thread-affinity) -- see their documentation for
-// details. Use a checker as a protected member instead of inheriting from
-// NonThreadSafe if you need subclasses to have access.
-#if DCHECK_IS_ON()
-typedef NonThreadSafeImpl NonThreadSafe;
-#else
-typedef NonThreadSafeDoNothing NonThreadSafe;
-#endif  // DCHECK_IS_ON()
-
-}  // namespace base
-
-#endif  // BASE_THREADING_NON_THREAD_SAFE_H_