Force fullscreen on Chrome OS

Am I able to force game being started in fullscreen mode?

I use custom GUI system that detects resolution and aspect ration when the game starts. When the resolution or aspect ratio changes in the middle of playing, the GUI gets broken.

It is not the answer you are looking for. But it is your and your custom GUI system problem. Android N allows freeform windows and you do not have any other option other than adopt. Why does it get messed up in the first place? You can check the last frame resolution and if the change occurs, then rebuild GUI.

Also, the whole android apps on chrome os thing is just android layer on top of chrome OS. And unity does not have an idea if it is running on android or chrome os.