Make callback.h include callback_forward.h and add comments about forward declaration.

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) 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_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.
+
 // New, super-duper, unified Callback system.  This will eventually replace
 // NewRunnableMethod, NewRunnableFunction, CreateFunctor, and CreateCallback
 // systems currently in the Chromium code base.
 //
 // WHAT IS THIS:
 //
 // The templated Callback class is a generalized function object. Together
 // with the Bind() function in bind.h, they provide a type-safe method for
 // performing currying of arguments, and creating a "closure."
 //
@@ skipping 195 matching lines @@
 
 namespace base {
 
 // First, we forward declare the Callback class template. This informs the
 // compiler that the template only has 1 type parameter which is the function
 // signature that the Callback is representing.
 //
 // After this, create template specializations for 0-$(MAX_ARITY) parameters. Note that
 // even though the template typelist grows, the specialization still
 // only has one type: the function signature.
+//
+// If you are thinking of forward declaring Callback in your own header file,
+// please include "base/callback_forward.h" instead.
 template <typename Sig>
 class Callback;
 
 
 $range ARITY 0..MAX_ARITY
 $for ARITY [[
 $range ARG 1..ARITY
 
 $if ARITY == 0 [[
 template <typename R>
@@ skipping 51 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