Bug 1838670 - Match parent selector like :scope. r=zrhoffman

https://drafts.csswg.org/css-nesting-1/#nest-selector:

    When used in any other context, it represents the same elements as
    :scope in that context (unless otherwise defined).

Differential Revision: https://phabricator.services.mozilla.com/D181119

servo/components/selectors/matching.rs

@@ -859,9 +859,7 @@ where
                         .nest(|context| matches_complex_selector(selector.iter(), element, context))
                 })
         },
-        // These should only work at parse time, should be replaced with :is() at CascadeData build
-        // time.
-        Component::ParentSelector => false,
+        Component::ParentSelector |
         Component::Scope => match context.shared.scope_element {
             Some(ref scope_element) => element.opaque() == *scope_element,
             None => element.is_root(),

testing/web-platform/meta/css/css-nesting/top-level-is-scope.html.ini

@@ -1,6 +0,0 @@
-[top-level-is-scope.html]
-  [& as direct ancestor]
-    expected: FAIL
-
-  [& matches scoped element only, not everything]
-    expected: FAIL