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",
 )}
 

components/style/values/specified/ui.rs

@@ -141,6 +141,11 @@ impl Parse for ScrollbarColor {
     }
 }
 
+fn in_ua_sheet(context: &ParserContext) -> bool {
+    use crate::stylesheets::Origin;
+    context.stylesheet_origin == Origin::UserAgent
+}
+
 /// The specified value for the `user-select` property.
 ///
 /// https://drafts.csswg.org/css-ui-4/#propdef-user-select
@@ -167,5 +172,11 @@ pub enum UserSelect {
     All,
     /// Like `text`, except that it won't get overridden by ancestors having
     /// `all`.
+    ///
+    /// FIXME(emilio): This only has one use in contenteditable.css, can we find
+    /// a better way to do this?
+    ///
+    /// See bug 1181130.
+    #[parse(condition = "in_ua_sheet")]
     MozText,
 }