`WP_Filesystem_Base->abspath()` should use the `WPINC` constant

Props wojtek.szkutnik
See #14157.

Built from https://develop.svn.wordpress.org/trunk@28904


git-svn-id: http://core.svn.wordpress.org/trunk@28703 1a063a9b-81f0-0310-95a4-ce76da25c4cd
`wlwmanifest_link()` should use the `WPINC` constant

Props wojtek.szkutnik
See #14157.

Built from https://develop.svn.wordpress.org/trunk@28905


git-svn-id: http://core.svn.wordpress.org/trunk@28704 1a063a9b-81f0-0310-95a4-ce76da25c4cd