Replace non-functional 'wp_audio_shortcode_handler' and 'wp_video_shortcode_handler' filters with 'wp_audio_shortcode_override' and 'wp_video_shortcode_override'.

props ericlewis, SergeyBiryukov.
fixes #25313.
Built from https://develop.svn.wordpress.org/trunk@25484


git-svn-id: http://core.svn.wordpress.org/trunk@25405 1a063a9b-81f0-0310-95a4-ce76da25c4cd