Core Update: Fix a issue which caused automatic upgrades from 2.7 to 3.5+ to...

Core Update: Fix a issue which caused automatic upgrades from 2.7 to 3.5+ to fail. Pre-2.7 WP_Filesystem::wp_content_dir() returned unslashed paths. Introduced in [22227]. See #23177


git-svn-id: https://core.svn.wordpress.org/trunk@23297 1a063a9b-81f0-0310-95a4-ce76da25c4cd