IDL: Treat undefined as missing for optional arguments with defaults

LayoutTests/fast/dom/TreeWalker/TreeWalker-basic.html

Index: LayoutTests/fast/dom/TreeWalker/TreeWalker-basic.html
diff --git a/LayoutTests/fast/dom/TreeWalker/TreeWalker-basic.html b/LayoutTests/fast/dom/TreeWalker/TreeWalker-basic.html
index 7c1438320e472c83262bff8dec1cc106513e5b04..1c1411c7d00841d958030fdf1561ceabd7436cff 100644
--- a/LayoutTests/fast/dom/TreeWalker/TreeWalker-basic.html
+++ b/LayoutTests/fast/dom/TreeWalker/TreeWalker-basic.html
@@ -35,10 +35,8 @@ function createSampleDOM()
     return div;
 }
 
-test(function ()
+function check_walker(walker, root)
 {
-    var root = createSampleDOM();
-    var walker = document.createTreeWalker(root);
     assert_equals(walker.toString(), '[object TreeWalker]');
     assert_equals(walker.root, root);
     assert_equals(walker.whatToShow, 0xFFFFFFFF);
@@ -54,7 +52,21 @@ test(function ()
     assert_idl_attribute(walker, 'nextSibling');
     assert_idl_attribute(walker, 'previousNode');
     assert_idl_attribute(walker, 'nextNode');
-}, 'Construct a TreeWalker by document.createTreeWalker().');
+}
+
+test(function ()
+{
+    var root = createSampleDOM();
+    var walker = document.createTreeWalker(root);
+    check_walker(walker, root);
+}, 'Construct a TreeWalker by document.createTreeWalker(root).');
+
+test(function ()
+{
+    var root = createSampleDOM();
+    var walker = document.createTreeWalker(root, undefined, undefined);
+    check_walker(walker, root);
+}, 'Construct a TreeWalker by document.createTreeWalker(root, undefined, undefined).');
 
 test(function ()
 {