After [29543], the iframe sandboxes for media need to load Open Sans. Playlists look aggressively bad without it. Adds font styles for the body.

See #29048.

Built from https://develop.svn.wordpress.org/trunk@29564


git-svn-id: http://core.svn.wordpress.org/trunk@29338 1a063a9b-81f0-0310-95a4-ce76da25c4cd