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

base/hash_tables.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.
 //
 
 //
 // Deal with the differences between Microsoft and GNU implemenations
 // of hash_map. Allows all platforms to use |base::hash_map| and
 // |base::hash_set|.
 //  eg:
 //   base::hash_map<int> my_map;
 //   base::hash_set<int> my_set;
 //
 // NOTE: It is an explicit non-goal of this class to provide a generic hash
 // function for pointers.  If you want to hash a pointers to a particular class,
 // please define the template specialization elsewhere (for example, in its
 // header file) and keep it specific to just pointers to that class.  This is
 // because identity hashes are not desirable for all types that might show up
 // in containers as pointers.
 
 #ifndef BASE_HASH_TABLES_H_
 #define BASE_HASH_TABLES_H_
-#pragma once
 
 #include "build/build_config.h"
 
 #include "base/string16.h"
 
 #if defined(COMPILER_MSVC)
 #include <hash_map>
 #include <hash_set>
 
 #define BASE_HASH_NAMESPACE stdext
@@ skipping 77 matching lines @@
 #else  // COMPILER
 #error define BASE_HASH_NAMESPACE for your compiler
 #endif  // COMPILER
 
 namespace base {
 using BASE_HASH_NAMESPACE::hash_map;
 using BASE_HASH_NAMESPACE::hash_set;
 }
 
 #endif  // BASE_HASH_TABLES_H_