GridLayout: Hide panels with zero width or zero height

If you don't hide them, IE will give them scrollbars. Even
setting to 0%x0% doesn't work, that still renders as a small
square in IE.

Hide with visibility: hidden; rather than display: none;
so we don't mess with the layout unnecessarily.

Change-Id: I1951b73e929a942519c0b7bd6332a8e70e1933c6