Sync the DOM traversal interfaces with the spec

Source/core/dom/NodeIterator.idl

Index: Source/core/dom/NodeIterator.idl
diff --git a/Source/core/dom/NodeIterator.idl b/Source/core/dom/NodeIterator.idl
index b420e7c3f77d0ae784238bdf0939d1888e5875d6..bca3510017df0eb049e4cb4168015f2b999a011c 100644
--- a/Source/core/dom/NodeIterator.idl
+++ b/Source/core/dom/NodeIterator.idl
@@ -18,18 +18,20 @@
  * Boston, MA 02110-1301, USA.
  */
 
-// Introduced in DOM Level 2
+// https://dom.spec.whatwg.org/#interface-nodeiterator
+
 [
     SetWrapperReferenceTo(NodeFilter filter),
     WillBeGarbageCollected,
 ] interface NodeIterator {
     readonly attribute Node root;
-    readonly attribute unsigned long whatToShow;
-    readonly attribute NodeFilter filter;
     readonly attribute Node referenceNode;
     readonly attribute boolean pointerBeforeReferenceNode;
+    readonly attribute unsigned long whatToShow;
+    readonly attribute NodeFilter? filter;
+
+    [RaisesException] Node? nextNode();
+    [RaisesException] Node? previousNode();
 
-    [RaisesException] Node nextNode();
-    [RaisesException] Node previousNode();
     [DeprecateAs=NodeIteratorDetach] void detach();
 };