Make StyleSheetList.item() index argument mandatory

Make StyleSheetList.item() index argument mandatory to match the specification:
http://www.w3.org/TR/2000/CR-DOM-Level-2-20000510/stylesheets.html#StyleSheets-StyleSheetList

This behavior is consistent with Firefox 23. The argument is however optional
in IE10, which is uncommon for an indexed getter.

BUG=

Review URL: https://chromiumcodereview.appspot.com/23629011

git-svn-id: svn://svn.chromium.org/blink/trunk@157054 bbb929c8-8fbe-4397-9dbb-9b2b20218538