Fix notices in wp_iframe() and iframe_header() by referencing...

Fix notices in wp_iframe() and iframe_header() by referencing $wp_htmltag_class as a global. See r18975 and #18314.


git-svn-id: https://core.svn.wordpress.org/trunk@18977 1a063a9b-81f0-0310-95a4-ce76da25c4cd