The index argument for HTMLCollection item() api should not be optional

Source/core/html/HTMLCollection.idl

Index: Source/core/html/HTMLCollection.idl
diff --git a/Source/core/html/HTMLCollection.idl b/Source/core/html/HTMLCollection.idl
index 85c2d5acb74e618a2c899948fed011e55cac5c1a..b285517acb0dfd89fad3e990acd8a4ddeb6e7203 100644
--- a/Source/core/html/HTMLCollection.idl
+++ b/Source/core/html/HTMLCollection.idl
@@ -27,7 +27,6 @@
     WillBeGarbageCollected,
 ] interface HTMLCollection {
     readonly attribute unsigned long length;
-    // FIXME: The index argument should not be optional.
-    getter Element? item([Default=Undefined] optional unsigned long index);
+    getter Element? item(unsigned long index);
     getter Element? namedItem(DOMString name);
 };