style: Allow user-select: -moz-text on user-agent stylesheets only.

It's only used in contenteditable.css, and same usage in comm-central. That
sheet is loaded as a ua sheet so let's restrict it to that. No relevant
external usage either. This value was introduced in bug 1181130.

Differential Revision: https://phabricator.services.mozilla.com/D11584

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

@@ -39,7 +39,6 @@ ${helpers.predefined_type(
     gecko_ffi_name="mUserSelect",
     alias="-webkit-user-select",
     animation_value_type="discrete",
-    needs_context=False,
     spec="https://drafts.csswg.org/css-ui-4/#propdef-user-select",
 )}