Tried ChromeOS for the first time a month or so back.  I found the Dvorak option.  Search settings for Dvorak.  Switch between layouts with CTRL+SPACE.  All easy.

Then after a reset/update, with a ChromeOS version from June 2017, I got into a bit of a mess.  I selected the Dvorak US keyboard and then lost the UK Qwerty one.   Something had changed.  And accessing the settings became more esoteric.  I think to remedy, I added the British language, and then the UK Qwerty option became available again as an input option.  Certainly not as straight forward as my first experience.

Anyway; I did notice at the same time that Colemak was there as an input option.