win32: scrollbars do not always appear when scrollpane first is shown

On win32, when you first bring up ScrollPaneTest in src/share/test/awt,
the top scrollpane (the one with the buttons) does not display any
scrollbars even though the entire panel of buttons is not visible.
If you resize the window, the scrollpane seems to map the scrollbars.

This is reproducible on winNT running 1.1beta3.