Most keyboard commands work just like the official Chrome and Chromium browsers.
To shut down the OS, press the power button momentarily.1)
You can also go to the terminal and type in:
sudo shutdown -h now
That will initiate an immediate shutdown.
Press F8 in ChromeOS to see complete list of keyboard commands (although they are not all currently supported)
Diagrams found in C-ROOT/usr/share/chronos-assets/images