This is a problem of the test itself, we are using an overflow that is not actually scrollable, but that shows scrollbar troughs when not using overlay scrollbars because it's marked as scrollable. It would be easier to simply use the main frame scrollbars to test that and it will work no matter of scrollbars are overlay or not.
Created attachment 276465 [details] Patch
Committed r199653: <http://trac.webkit.org/changeset/199653>