Remove #pragma once. Just from base/ for now.

base/threading/thread_checker_impl.h

 // Copyright (c) 2011 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_THREAD_CHECKER_IMPL_H_
 #define BASE_THREADING_THREAD_CHECKER_IMPL_H_
-#pragma once
 
 #include "base/base_export.h"
 #include "base/synchronization/lock.h"
 #include "base/threading/platform_thread.h"
 
 namespace base {
 
 // Real implementation of ThreadChecker, for use in debug mode, or
 // for temporary use in release mode (e.g. to CHECK on a threading issue
 // seen only in the wild).
@@ skipping 17 matching lines @@
 
   mutable base::Lock lock_;
   // This is mutable so that CalledOnValidThread can set it.
   // It's guarded by |lock_|.
   mutable PlatformThreadId valid_thread_id_;
 };
 
 }  // namespace base
 
 #endif  // BASE_THREADING_THREAD_CHECKER_IMPL_H_