BUG ID: http://bugs.webkit.org/show_bug.cgi?id=5829 REGRESSION: cursor tracking and hit testing in scrollable block don't take borders into account

STEPS TO TEST:
1. Scroll down by clicking in the empty space below the scroll thumb.
2. Scroll up by clicking in the empty space above the scroll thumb.

TEST PASS: Scroll thumb will move back up to the top.

TEST FAIL: Scroll thumb will NOT move back up to the top.