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/longhands/ui.mako.rs

@@ -31,16 +31,15 @@ ${helpers.single_keyword(
     spec="https://drafts.csswg.org/css-scrollbars-1/#scrollbar-width"
 )}
 
-${helpers.single_keyword(
+${helpers.predefined_type(
     "-moz-user-select",
-    "auto text none all element elements toggle tri-state -moz-all -moz-text",
+    "UserSelect",
+    "computed::UserSelect::Auto",
     products="gecko",
-    alias="-webkit-user-select",
     gecko_ffi_name="mUserSelect",
-    gecko_enum_prefix="StyleUserSelect",
-    gecko_strip_moz_prefix=False,
-    aliases="-moz-none=none",
+    alias="-webkit-user-select",
     animation_value_type="discrete",
+    needs_context=False,
     spec="https://drafts.csswg.org/css-ui-4/#propdef-user-select",
 )}