Revert [20001]. Theme roots that are passed around, stored, and cached can be...

Revert [20001]. Theme roots that are passed around, stored, and cached can be valid as absolute paths or paths relative to wp-content. We'll have to patch this elsewhere. see #17597.

git-svn-id: https://core.svn.wordpress.org/trunk@20015 1a063a9b-81f0-0310-95a4-ce76da25c4cd