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/style/selector_map.rs

@@ -457,6 +457,7 @@ fn specific_bucket_for<'a>(
         //
         // Meanwhile taking the code path below is slower, but still correct.
         // Component::Slotted(ref selector) => find_bucket(selector.iter()),
+        Component::Host(Some(ref selector)) => find_bucket(selector.iter()),
         _ => Bucket::Universal
     }
 }