Make sure editor-buttons.css is loaded even if tiny_mce.js is used directly...

Make sure editor-buttons.css is loaded even if tiny_mce.js is used directly and the wp_theme skin is selected in the settings, see #17144

git-svn-id: https://core.svn.wordpress.org/trunk@19533 1a063a9b-81f0-0310-95a4-ce76da25c4cd