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

base/callback.h

Index: base/callback.h
diff --git a/base/callback.h b/base/callback.h
index 16e3ee60c51971f87bb914e9e52bf5fd9ddd3091..09c3ef74dfcb24a121322ab75547ce965f9ca62e 100644
--- a/base/callback.h
+++ b/base/callback.h
@@ -11,9 +11,13 @@
 #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.
@@ -229,6 +233,9 @@ namespace base {
 // After this, create template specializations for 0-7 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;