style: Apply selector flags to the shadow root

Because restyle events cannot be posted for non-element nodes like the
shadow root, a child's siblings are restyled directly if its parent has
NODE_HAS_SLOW_SELECTOR or NODE_HAS_SLOW_SELECTOR_LATER_SIBLINGS but that
parent is the shadow root.

Drive-by, but braces were also added to some single-line "if" statements
in RestyleManager.

Differential Revision: https://phabricator.services.mozilla.com/D172352

components/style/invalidation/element/element_wrapper.rs

@@ -238,7 +238,7 @@ where
         }
     }
 
-    fn set_selector_flags(&self, _flags: ElementSelectorFlags) {
+    fn apply_selector_flags(&self, _flags: ElementSelectorFlags) {
         debug_assert!(false, "Shouldn't need selector flags for invalidation");
     }