When a Chrome developer fixes something that needs to be backported to a branch the TPMs ask if it's been tested on canary. If you want to test a change yourself on canary you need to download an image, since we don't let you switch to canary in the settings menu. There's a magic crosh command ("live_in_a_coal_mine") to switch to canary but that command isn't discoverable and it's only available on dev-mode devices.

We don't want to make it too easy to get onto canary channel, since it could brick the device or break the updater. Also, we want to avoid issues with device ownership and enrollment. See the live_in_a_coal_mine CL https://chromium-review.googlesource.com/c/344670/ - we should still limit the actual channel switch to developer mode. However, we should at least mention it in the non-developer-mode UI to help developers discover how to switch.

Proposed UI for "Change channel" dialog (not in developer mode):

(*) Stable
( ) Beta
( ) Dev - unstable
Canary channel available in developer mode  <- new label

"Change channel" dialog (in developer mode):

(*) Stable
( ) Beta
( ) Dev - unstable
( ) Canary - may damage your device

Notes:
* This changes the dev channel text from "Developer - unstable" to "Dev - unstable". That's to help emphasize that "dev channel" and "developer mode" are unrelated. Our help center refers to "dev channel" not "developer channel" https://support.google.com/chromebook/answer/1086915?hl=en
* I'm aware canary channel does not *require* developer mode. However, that's a piece of complexity that doesn't belong in this UI. People who are able to find and download canary images are likely to know that they will work without dev-mode.
* I say "may damage" because that matches the boot screen "Chrome OS is missing or damaged" text, and I think it is both brief and sufficiently scary.

For implementation: Chrome OS webui already knows if the machine is in developer mode, since OOBE uses that information to show the "Enable debugging features" link.