The latest spec has renamed the regionOverflow property to regionOverset, and the overflow property value to overset: http://www.w3.org/TR/css3-regions/#changes . We need to also change this in Webkit.
You should close this as duplicate of https://bugs.webkit.org/show_bug.cgi?id=93256 :).
*** This bug has been marked as a duplicate of bug 93256 ***