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

base/threading/thread_checker.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_THREAD_CHECKER_H_
 #define BASE_THREADING_THREAD_CHECKER_H_
-#pragma once
 
 // Apart from debug builds, we also enable the thread checker in
 // builds with DCHECK_ALWAYS_ON so that trybots and waterfall bots
 // with this define will get the same level of thread checking as
 // debug bots.
 //
 // Note that this does not perfectly match situations where DCHECK is
 // enabled.  For example a non-official release build may have
 // DCHECK_ALWAYS_ON undefined (and therefore ThreadChecker would be
 // disabled) but have DCHECKs enabled at runtime.
@@ skipping 59 matching lines @@
 #else
 class ThreadChecker : public ThreadCheckerDoNothing {
 };
 #endif  // ENABLE_THREAD_CHECKER
 
 #undef ENABLE_THREAD_CHECKER
 
 }  // namespace base
 
 #endif  // BASE_THREADING_THREAD_CHECKER_H_