Generate geckolib setters for properties with simple predefined types.

components/style/properties/data.py

@@ -46,10 +46,11 @@ class Keyword(object):
 
 class Longhand(object):
     def __init__(self, style_struct, name, derived_from=None, keyword=None,
-                 custom_cascade=False, experimental=False, internal=False,
+                 predefined_type=None, custom_cascade=False, experimental=False, internal=False,
                  need_clone=False, gecko_ffi_name=None):
         self.name = name
         self.keyword = keyword
+        self.predefined_type = predefined_type
         self.ident = to_rust_ident(name)
         self.camel_case = to_camel_case(self.ident)
         self.style_struct = style_struct

components/style/properties/helpers.mako.rs

@@ -17,7 +17,7 @@
 </%def>
 
 <%def name="predefined_type(name, type, initial_value, parse_method='parse', **kwargs)">
-    <%call expr="longhand(name, **kwargs)">
+    <%call expr="longhand(name, predefined_type=type, **kwargs)">
         #[allow(unused_imports)]
         use app_units::Au;
         pub type SpecifiedValue = specified::${type};