This is copied from the g + thread.
I have tried this and initially had some issues getting it to work, maybe my experience can help others to get this working:
For me I already had a crouton chroot running Ubuntu Trusty on 2 different Chromebook so I did not want to wipe and start over (after all in unix everything is a file).
so to update my chroots I did this after installing the chrome browser extension:
$ sudo sh ~/Downloads/crouton -u -e -t xiwi,extension -n trusty
this loads the new targets and the '-e' encrypts the chroot and forces me to make a root password for the Chromebook which seems like a good idea.
So on one machine this is all I had to do ... .If I run $ sudo startxfce4
it pops up a window, however for the other machine it would continue to open full screen.
the reason for this is wrong symbolic link at /etc/X11/xinit/xserverrc
basically if it is linked to /etc/crouton/xserverrc-x11 you get the full screen version, if it is linked to /etc/crouton/xserverrc-xiwi you get the windowed linux.
to change this simply:
$ ln -s /etc/crouton/xserverrc-[xiwi|x11] /etc/x11/xinit/xserverrc
edit: Google plus's markup syntax is jacked with no apparent way to escape a "-"; so the strikethrough is artifactual.
$ ln -s
(the brackets  with the | indicate choose one)
These computers are used by my kids and the linux partition is primarily used to play Minecraft. As there is not currently hardware graphics acceleration in the xiwi linux chrome window the performance is quite poor with Minecraft so I switched back to the xserverrc-x11 config. This will be much better once GPU acceleration is supported in window (whispers of which can be seen on the github PR), until then I guess we will have to <Ctl>+<alt>+<shift> + forward and back to use linux on our Chromebooks.