How to exit fullscreen in android chrome?
I don't think you can. The site is supporting the mobile-web-app-capable view which means Chrome displays it like an app, hides the address bar and it integrates into the recent apps menu as though it was a 'normal' app.
I don't believe there is a way to exit fullscreen when using a bookmark to a site like this.

For those who are unsure what the OP is referring to, http://weight.aerotwist.com/ is a site that supports this 'app' mode. If you browse to the site in Chrome and chose the menu option "Add to home screen" this adds a 'bookmark' to your home screen which when used launches the site in Chrome in app mode.

See here for more details.