Use variadic template for screen handler's AddCallback.

components/login/base_screen_handler_utils.cc

Index: components/login/base_screen_handler_utils.cc
diff --git a/components/login/base_screen_handler_utils.cc b/components/login/base_screen_handler_utils.cc
index 782c0643327c8306da94a879310fa6028b66133a..6454cdf87b2a1d70dd8f899ff0c4ef31352852dd 100644
--- a/components/login/base_screen_handler_utils.cc
+++ b/components/login/base_screen_handler_utils.cc
@@ -76,11 +76,4 @@ base::StringValue MakeValue(const base::string16& v) {
   return base::StringValue(v);
 }
 
-void CallbackWrapper0(base::Callback<void()> callback,
-                      const base::ListValue* args) {
-  DCHECK(args);
-  DCHECK(args->empty());
-  callback.Run();
-}
-
 }  // namespace login