Make CSSValueList.item() index argument mandatory

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

This behavior is consistent with Firefox 23. I wasn't able to test on IE10.
It is confusing what the return value is supposed to be if the index is
omitted.

BUG=

Review URL: https://chromiumcodereview.appspot.com/23526011

git-svn-id: svn://svn.chromium.org/blink/trunk@157057 bbb929c8-8fbe-4397-9dbb-9b2b20218538