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

base/callback.h.pump

Index: base/callback.h.pump
diff --git a/base/callback.h.pump b/base/callback.h.pump
index aa43ad525bc3137ac60e4c6685e72c71815577f8..76473890378b3814f48acc8261db7726532b5507 100644
--- a/base/callback.h.pump
+++ b/base/callback.h.pump
@@ -16,9 +16,13 @@ $var MAX_ARITY = 7
 #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.
@@ -234,6 +238,9 @@ namespace base {
 // 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;