style: Move user-select outside of mako.

This is the first step to unprefix user-select.

This has no behavior change, it's just a nicer way to do the same thing which
allows us to unship individual values more easily using parse(condition).

Differential Revision: https://phabricator.services.mozilla.com/D11580

components/style/properties/gecko.mako.rs

@@ -1429,6 +1429,7 @@ impl Clone for ${style_struct.gecko_struct_name} {
         "SVGWidth": impl_svg_length,
         "Transform": impl_transform,
         "TransformOrigin": impl_transform_origin,
+        "UserSelect": impl_simple,
         "url::UrlOrNone": impl_css_url,
     }