Move Chrome OS features into //chromeos/constants/chromeos_features.h

Some of them were left in //chromeos/constants/chromeos_switches.h for
historical reasons.

Fix a few IMYU issues.

Bug: none
Test: bots
Change-Id: I1480c141a76a69c4153d6d1019530e9efcaa80e2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1731957
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Reviewed-by: Mihai Sardarescu <msarda@chromium.org>
Auto-Submit: James Cook <jamescook@chromium.org>
Commit-Queue: James Cook <jamescook@chromium.org>
Cr-Commit-Position: refs/heads/master@{#684048}