Internal Chrome instructions are here:

https://companydoc.corp.google.com/company/teams/chrome/chrome_build_instructions.md

This page lists instructions for internal Chrome checkouts for various OSs (Linux, Windows, Android, etc). However, there is no page for CrOS development.

https://chromium.googlesource.com/chromiumos/docs/+/master/simple_chrome_workflow.md also links to that page as well.