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

base/file_path.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.
 
 // FilePath is a container for pathnames stored in a platform's native string
 // type, providing containers for manipulation in according with the
 // platform's conventions for pathnames.  It supports the following path
 // types:
 //
 //                   POSIX            Windows
@@ skipping 81 matching lines @@
 //    equivalent.  Since the OS treats these paths specially, FilePath needs
 //    to do the same.  Since Windows can use either / or \ as the separator,
 //    FilePath treats c://, c:\\, //, and \\ all equivalently.
 //    Reference:
 //    The Old New Thing, "Why is a drive letter permitted in front of UNC
 //    paths (sometimes)?", available at:
 //    http://blogs.msdn.com/oldnewthing/archive/2005/11/22/495740.aspx
 
 #ifndef BASE_FILE_PATH_H_
 #define BASE_FILE_PATH_H_
-#pragma once
 
 #include <stddef.h>
 #include <string>
 #include <vector>
 
 #include "base/base_export.h"
 #include "base/compiler_specific.h"
 #include "base/hash_tables.h"
 #include "base/string16.h"
 #include "base/string_piece.h"  // For implicit conversions.
@@ skipping 312 matching lines @@
 
 inline size_t hash_value(const FilePath& f) {
   return hash_value(f.value());
 }
 
 #endif  // COMPILER
 
 }  // namespace BASE_HASH_NAMESPACE
 
 #endif  // BASE_FILE_PATH_H_