Add a CallbackRegistry class to base/ to manage callbacks

base/callback_internal.h

Index: base/callback_internal.h
diff --git a/base/callback_internal.h b/base/callback_internal.h
index 5993824a5a2a28d1d7ccb643ed60fdf24d54c8be..cd00d74c89269e6aa78ac2124275f20a369eb88e 100644
--- a/base/callback_internal.h
+++ b/base/callback_internal.h
@@ -20,6 +20,8 @@ class ScopedVector;
 namespace base {
 namespace internal {
 
+class CallbackListImpl;
+
 // BindStateBase is used to provide an opaque handle that the Callback
 // class can use to represent a function object with bound arguments.  It
 // behaves as an existential type that is used by a corresponding
@@ -65,6 +67,9 @@ class BASE_EXPORT CallbackBase {
 
   scoped_refptr<BindStateBase> bind_state_;
   InvokeFuncStorage polymorphic_invoke_;
+
+ private:
+  friend class CallbackListImpl;
 };
 
 // This is a typetraits object that's used to take an argument type, and