Use callback_forward.h instead of callback.h where possible.

base/message_loop.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_MESSAGE_LOOP_H_
 #define BASE_MESSAGE_LOOP_H_
 #pragma once
 
 #include <queue>
 #include <string>
 
 #include "base/base_export.h"
 #include "base/basictypes.h"
-#include "base/callback.h"
+#include "base/callback_forward.h"
 #include "base/location.h"
 #include "base/memory/ref_counted.h"
 #include "base/message_loop_proxy.h"
 #include "base/message_pump.h"
 #include "base/observer_list.h"
 #include "base/pending_task.h"
 #include "base/synchronization/lock.h"
 #include "base/task.h"
 #include "base/tracking_info.h"
 #include "base/time.h"
@@ skipping 641 matching lines @@
 #endif  // defined(OS_POSIX)
 };
 
 // Do not add any member variables to MessageLoopForIO!  This is important b/c
 // MessageLoopForIO is often allocated via MessageLoop(TYPE_IO).  Any extra
 // data that you need should be stored on the MessageLoop's pump_ instance.
 COMPILE_ASSERT(sizeof(MessageLoop) == sizeof(MessageLoopForIO),
                MessageLoopForIO_should_not_have_extra_member_variables);
 
 #endif  // BASE_MESSAGE_LOOP_H_