NodeList.item() does not behave according to specification

NodeList.item() does not behave according to specification:
http://dom.spec.whatwg.org/#nodelist

Namely, we should not throw if the index is negative (meaning we should
drop the [IsIndex] extended attribute) and the argument should be
mandatory.

This behavior is consistent with both IE10 and Firefox.

BUG=237739

Review URL: https://chromiumcodereview.appspot.com/21766002

git-svn-id: svn://svn.chromium.org/blink/trunk@155514 bbb929c8-8fbe-4397-9dbb-9b2b20218538