make the xpinstall manager a weak observer by inheriting from nsSupportsWeakReference - prevents xpinstall manager from lingering around until the app shuts down long after its usefulness has expired.
r=danm, sr=dveditz, a=chofmann
git-svn-id: svn://10.0.0.236/trunk@153931 18797224-902f-48f8-a5cc-f745e15eee43