Scrollbar Hittest2

BUG ID: http://bugs.webkit.org/show_bug.cgi?id=6149 REGRESSION (WebCore-417.5): horizontal scrollbar in overflow with top border doesn't receive mouse events

TO TEST:
Scroll right by clicking in the empty space to the right of the scroll thumb.

TEST PASS: Scroll thumb will move to the right.

TEST FAIL: Scroll thumb will NOT move at all.