Bug 1600436 - Fix servo build.

Depends on D55357

Differential Revision: https://phabricator.services.mozilla.com/D55358

--HG--
extra : moz-landing-system : lando

servo/components/style/servo/selector_parser.rs

@@ -700,6 +700,14 @@ impl ElementSnapshot for ServoElementSnapshot {
         false
     }
 
+    fn exported_part(&self, _: &Atom) -> Option<Atom> {
+        None
+    }
+
+    fn imported_part(&self, _: &Atom) -> Option<Atom> {
+        None
+    }
+
     fn has_class(&self, name: &Atom, case_sensitivity: CaseSensitivity) -> bool {
         self.get_attr(&ns!(), &local_name!("class"))
             .map_or(false, |v| {