export boxedness of types from longhand_properties_idents

The only piece of information about longhand SpecifiedValue types that
we don't have outside of the properties module is whether the types are
boxed or not.  Provide that information in the
longhand_properties_idents macro so we can move
test_size_of_specified_values outside of the properties module.

components/style/lib.rs

@@ -171,7 +171,7 @@ pub mod gecko_properties {
 }
 
 macro_rules! reexport_computed_values {
-    ( $( $name: ident )+ ) => {
+    ( $( { $name: ident, $boxed: expr } )+ ) => {
         /// Types for [computed values][computed].
         ///
         /// [computed]: https://drafts.csswg.org/css-cascade/#computed