style: Use a more similar representation in Rust and C++ for grid lines.

Option<> is not FFI-safe, so if we want to use the same representation
everywhere we need to get rid of it. This also makes it take the same amount of
memory as the C++ representation, and it's not very complex, I'd think.

Differential Revision: https://phabricator.services.mozilla.com/D35195

components/style/properties/longhands/position.mako.rs

@@ -322,7 +322,6 @@ ${helpers.predefined_type(
             animation_value_type="discrete",
             spec="https://drafts.csswg.org/css-grid/#propdef-grid-%s-%s" % (kind, range),
             products="gecko",
-            boxed=True,
         )}
     % endfor