Fixes a crashy over-releasing bug in FF plugin.

We were failing to AddRef the topWin reference, which would cause random crashes later.

Patch by: me, jat (pair prog)
Review by: jat

git-svn-id: https://google-web-toolkit.googlecode.com/svn/trunk@7206 8db76d5a-ed1c-0410-87a9-c151d255dfc7