style: Implement the functional :host(..) selector.

We could invalidate in a slightly more fine-grained way, but I don't think it's
worth the churn vs. keeping the special-cases minimal.

Bug: 1452640
Reviewed-by: xidorn
MozReview-Commit-ID: 5DkQrgwg9GW

components/selectors/builder.rs

@@ -314,7 +314,7 @@ fn complex_selector_specificity<Impl>(mut iter: slice::Iter<Component<Impl>>)
             Component::FirstChild | Component::LastChild |
             Component::OnlyChild | Component::Root |
             Component::Empty | Component::Scope |
-            Component::Host |
+            Component::Host(..) |
             Component::NthChild(..) |
             Component::NthLastChild(..) |
             Component::NthOfType(..) |