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

base/value_conversions.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_VALUE_CONVERSIONS_H_
 #define BASE_VALUE_CONVERSIONS_H_
-#pragma once
 
 // This file contains methods to convert things to a |Value| and back.
 
 #include "base/base_export.h"
 
 class FilePath;
 
 namespace base {
 
 class TimeDelta;
 class StringValue;
 class Value;
 
 // The caller takes ownership of the returned value.
 BASE_EXPORT StringValue* CreateFilePathValue(const FilePath& in_value);
 BASE_EXPORT bool GetValueAsFilePath(const Value& value, FilePath* file_path);
 
 BASE_EXPORT StringValue* CreateTimeDeltaValue(const TimeDelta& time);
 BASE_EXPORT bool GetValueAsTimeDelta(const Value& value, TimeDelta* time);
 
 }  // namespace
 
 #endif  // BASE_VALUE_CONVERSIONS_H_