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

base/callback.h.pump

 $$ This is a pump file for generating file templates.  Pump is a python
 $$ script that is part of the Google Test suite of utilities.  Description
 $$ can be found here:
 $$
 $$ http://code.google.com/p/googletest/wiki/PumpManual
 $$
 
 $$ See comment for MAX_ARITY in base/bind.h.pump.
 $var MAX_ARITY = 7
 
 // 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_CALLBACK_H_
 #define BASE_CALLBACK_H_
-#pragma once
 
 #include "base/callback_forward.h"
 #include "base/callback_internal.h"
 #include "base/template_util.h"
 
 // NOTE: Header files that do not require the full definition of Callback or
 // Closure should #include "base/callback_forward.h" instead of this file.
 
 // WHAT IS THIS:
 //
@@ skipping 275 matching lines @@
 
 ]]  $$ for ARITY
 
 // Syntactic sugar to make Callbacks<void(void)> easier to declare since it
 // will be used in a lot of APIs with delayed execution.
 typedef Callback<void(void)> Closure;
 
 }  // namespace base
 
 #endif  // BASE_CALLBACK_H