window-stack-bridge should only run on Unity

[Alkis Georgopoulos]

I think window-stack-bridge should only run on Unity, not e.g. on gnome-flashback.

The relevant upstart files are:

For window-stack-bridge:
$ grep 'start on' /usr/share/upstart/sessions/window-stack-bridge.conf
start on started dbus

For hud, which is fine:
$ grep 'start on' /usr/share/upstart/sessions/hud.conf
# start on dbus-activation com.canonical.hud
start on started dbus and ((xsession SESSION=ubuntu-touch) or (xsession SESSION=ubuntu-touch-surfaceflinger) or (xsession SESSION=ubuntu))

And, proof that it's running on gnome-flashback, Trusty:
$ echo $GDMSESSION
gnome-fallback

$ ps -fC window-stack-bridge
UID PID PPID C STIME TTY TIME CMD
alkisg 1471 1238 0 06:04 ? 00:00:00 /usr/lib/i386-linux-gnu/hud/window-stack-bridge

$ dpkg-query -W hud
hud 13.10.1+14.04.20140108.is.13.10.1+14.04.20131205-0ubuntu1