Fix scrollbar handles so it follows cursor position when on a page

When a webpage contains a scrollbar as part of an element inside it
then the scrollbar handle should be in sync with the cursor when
the handle is moved.

Task-number: QTBUG-38360
Change-Id: I1f5474ad9b5e8dc232b307f4c5927833577afe1b
Reviewed-by: Allan Sandfeld Jensen <allan.jensen@digia.com>