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

base/threading/thread_collision_warner.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_COLLISION_WARNER_H_
 #define BASE_THREADING_THREAD_COLLISION_WARNER_H_
-#pragma once
 
 #include <memory>
 
 #include "base/atomicops.h"
 #include "base/base_export.h"
 #include "base/compiler_specific.h"
 
 // A helper class alongside macros to be used to verify assumptions about thread
 // safety of a class.
 //
@@ skipping 218 matching lines @@
   // Here only for class unit tests purpose, during the test I need to not
   // DCHECK but notify the collision with something else.
   AsserterBase* asserter_;
 
   DISALLOW_COPY_AND_ASSIGN(ThreadCollisionWarner);
 };
 
 }  // namespace base
 
 #endif  // BASE_THREADING_THREAD_COLLISION_WARNER_H_