Implement Scrollable interface for viewport resize rules in Swing RootPanel.

I noticed when Flying Saucer lays out a page within a JScrollPane, the viewport dimensions wrer often lagging behind the JScrollPane component size. This was causing scroll bars to flicker when resizing a frame with a scrollable RootPanel in it (very noticeable when scroll bar policy is set to 'AS_NEEDED'). This patch fixes the problem by using the Scrollable interface to handle setting the viewport size.