Make MediaList API arguments mandatory

Make MediaList API arguments mandatory to match the specification:
http://www.w3.org/TR/DOM-Level-2-Style/stylesheets.html#StyleSheets-MediaList

This behavior is consistent with Firefox 23 and IE10. It is confusing what
the result is supposed to be when the arguments are omitted.

BUG=

Review URL: https://chromiumcodereview.appspot.com/23621013

git-svn-id: svn://svn.chromium.org/blink/trunk@157063 bbb929c8-8fbe-4397-9dbb-9b2b20218538