Always return an absolute path in get_raw_theme_root() and get_theme_roots().

These functions were changed in [15641] to avoid any calculations when only one theme directory is registered. However, the short-circuit ended up being relative to WP_CONTENT_DIR, rather than absolute. This broke situations where theme roots are outside the content directory (technically allowed), and made return values inconsistent, as when multiple roots were registered, absolute paths were always returned.

fixes #17597. see #20103. see #14911.



git-svn-id: https://core.svn.wordpress.org/trunk@20001 1a063a9b-81f0-0310-95a4-ce76da25c4cd