Add access modifier (`public`) to methods and members of `WP_Upgrader_Skin` and its subclasses.

See #27881, #22234.

Built from https://develop.svn.wordpress.org/trunk@28495


git-svn-id: http://core.svn.wordpress.org/trunk@28321 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Add access modifier (`public`) to methods and members of `WP_Upgrader` and its subclasses.

See #27881, #22234.

Built from https://develop.svn.wordpress.org/trunk@28496


git-svn-id: http://core.svn.wordpress.org/trunk@28322 1a063a9b-81f0-0310-95a4-ce76da25c4cd