style: Make svg:use use an actual shadow tree.

This fixes a couple fuzz bugs and prevents special-casing <svg:use> even more in
bug 1431255.

Unfortunately not as many hacks went away as I'd have hoped, since we still need
to match document rules, see the linked SVGWG issues.

But blocks_ancestor_combinators goes away, which is nice since it's on a very
hot path.

Bug: 1450250
Reviewed-by: heycam
Differential Revision: https://phabricator.services.mozilla.com/D2154

MozReview-Commit-ID: C4mthjoSNFh

components/selectors/matching.rs

@@ -418,10 +418,6 @@ where
     match combinator {
         Combinator::NextSibling | Combinator::LaterSibling => element.prev_sibling_element(),
         Combinator::Child | Combinator::Descendant => {
-            if element.blocks_ancestor_combinators() {
-                return None;
-            }
-
             match element.parent_element() {
                 Some(e) => return Some(e),
                 None => {},

components/selectors/tree.rs

@@ -129,11 +129,4 @@ pub trait Element: Sized + Clone + Debug {
     fn ignores_nth_child_selectors(&self) -> bool {
         false
     }
-
-    /// Return true if we want to stop lookup ancestor of the current
-    /// element while matching complex selectors with descendant/child
-    /// combinator.
-    fn blocks_ancestor_combinators(&self) -> bool {
-        false
-    }
 }