Make the before() and after() methods in Bulk_Plugin_Upgrader_Skin and Bulk_Theme_Upgrader_Skin declarations match Bulk_Upgrader_Skin. Eliminates strict standards warning.

Props charlestonsw
fixes #23807


git-svn-id: http://core.svn.wordpress.org/trunk@23749 1a063a9b-81f0-0310-95a4-ce76da25c4cd