style: Simplify NAC setup

Make all UA widgets also NAC.

Keep the UA widget flag but break at anonymous subtree boundaries, so
that only nodes inside the UA widget directly (and not NAC from those)
get the flag.

This is important because two callers depend on this difference:

  * The style system, since we still want to match content rules from
    stylesheets in the UA widget. We also match user rules, which is a
    bit sketchy, but that was the previous behavior, will file a
    follow-up for that.

  * The reflector code, since we want the scope for UA widgets to not
    include the NAC nodes inside that UA widget. nsINode::IsInUAWidget
    got it wrong.

After this patch, ChromeOnlyAccess is equivalent to
IsInNativeAnonymousSubtree, so we should probably unify the naming.
That's left for a follow-up patch because I don't have a strong
preference.

Differential Revision: https://phabricator.services.mozilla.com/D174310

components/style/traversal.rs

@@ -422,7 +422,7 @@ pub fn recalc_style_at<E, D, F>(
     if let Some(restyle_kind) = restyle_kind {
         child_restyle_requirement = compute_style(traversal_data, context, element, data, restyle_kind);
 
-        if element.is_in_native_anonymous_subtree() {
+        if !element.matches_user_and_content_rules() {
             // We must always cascade native anonymous subtrees, since they
             // may have pseudo-elements underneath that would inherit from the
             // closest non-NAC ancestor instead of us.