Implement Element#closest

fixes #4603

- Add definition to the Element.webidl and implementation to element.rs.
- Create inclusive_ancestors helper in NodeHelpers
- Update test expectations

components/script/dom/webidls/Element.webidl

@@ -44,6 +44,9 @@ interface Element : Node {
   boolean hasAttribute(DOMString name);
   boolean hasAttributeNS(DOMString? namespace, DOMString localName);
 
+  [Throws]
+  Element? closest(DOMString selectors);
+
   [Throws]
   boolean matches(DOMString selectors);