Make CSSRuleList.item() index argument mandatory

Make CSSRuleList.item() index argument mandatory to match the specification:
http://www.w3.org/TR/DOM-Level-2-Style/css.html#CSS-CSSRuleList

This behavior is consistent with Firefox 23 and IE10. It is confusing what
should be returned if the index is omitted.

BUG=
R=arv@chromium.org, tkent@chromium.org

Review URL: https://codereview.chromium.org/23682005

git-svn-id: svn://svn.chromium.org/blink/trunk@157069 bbb929c8-8fbe-4397-9dbb-9b2b20218538