what is called "full screen" under X11 is really a client message sent from the application to the window manager, which will then resize the window and hide the window frame; if there is no window manager, there is nothing to honour the policy. even the geometry request goes through the window manager: the toolkit can but ask.
the question is: are you modifying the Chromium code base for your application, or are you just launching the application itself? if you have access to the windowing system code you can get the screen size and set the window geometry yourself; see the GdkScreen
API:
https://developer.gnome.org/gdk2/stable/GdkScreen.html
I would still suggest you run a small window manager; running without one degrades the functionality of any application. you can use a simple one, like twm:
http://en.wikipedia.org/wiki/Twm
or a slightly more complex, and yet very plain one, like Metacity: