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

base/id_map.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.
 
 #ifndef BASE_ID_MAP_H_
 #define BASE_ID_MAP_H_
-#pragma once
 
 #include <set>
 
 #include "base/basictypes.h"
 #include "base/hash_tables.h"
 #include "base/logging.h"
 #include "base/threading/non_thread_safe.h"
 
 // Ownership semantics - own pointer means the pointer is deleted in Remove()
 // & during destruction
@@ skipping 192 matching lines @@
 
   HashTable data_;
 
   // See description above setter.
   bool check_on_null_data_;
 
   DISALLOW_COPY_AND_ASSIGN(IDMap);
 };
 
 #endif  // BASE_ID_MAP_H_