Use a time constant in get_theme_feature_list() and File_Upload_Upgrader.

props GaVrA.
fixes #28124.
Built from https://develop.svn.wordpress.org/trunk@28253


git-svn-id: http://core.svn.wordpress.org/trunk@28081 1a063a9b-81f0-0310-95a4-ce76da25c4cd