Use declarative shadow DOM polyfill for :nth-child(... of <selector>) in shadow root reftest

Although the polyfill script was already included, it was not called
(and we still need it until bug 1712140 is resolved).

Differential Revision: https://phabricator.services.mozilla.com/D171937

bugzilla-url: https://bugzilla.mozilla.org/show_bug.cgi?id=1821269
gecko-commit: 4caae26ec168abb38da7a1a57b96641b53247083
gecko-reviewers: emilio

Author: zrhoffman

css/selectors/invalidation/nth-child-in-shadow-root.html

@@ -17,6 +17,7 @@
   </template>
 </div>
 <script>
+  polyfill_declarative_shadow_dom(host);
   host.offsetTop;
   host.shadowRoot.getElementById('toggler').classList.toggle('foo');
 </script>

css/selectors/invalidation/nth-last-child-in-shadow-root.html

@@ -19,6 +19,7 @@
   </template>
 </div>
 <script>
+  polyfill_declarative_shadow_dom(host);
   host.offsetTop;
   host.shadowRoot.getElementById("toggler").classList.toggle("foo");
 </script>