Chat post format code cleanup.

see #23625. props obenland.

git-svn-id: http://core.svn.wordpress.org/trunk@23876 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Twenty Thirteen: update Chat post format styles to match markup changes in r23876. See #23869.

git-svn-id: http://core.svn.wordpress.org/trunk@23877 1a063a9b-81f0-0310-95a4-ce76da25c4cd