Return the correct style element for PopupPanels in FireFox on Mac, which uses an extra div.  The JavaDoc explains that all style should apply to the inner div.

Patch by: jlabanca
Review by: rjrjr
Issue: 3214



git-svn-id: https://google-web-toolkit.googlecode.com/svn/releases/1.6@4379 8db76d5a-ed1c-0410-87a9-c151d255dfc7