Fixes issue where user gets stuck at login screen after trying to close the app if previously they had to first login to access the Customizer. Prevents `WP_Customize_Manager::get_return_url()` from using `wp-login.php` as a referer. Props chandrapatel. See #32637. Fixes #35355. git-svn-id: https://develop.svn.wordpress.org/trunk@36261 602fd350-edb4-49c9-b593-d223f7449a82