Default is_chromeos_device to false.

It's explicitly true for the ebuild (and consequently simple chrome):
https://chromium.git.corp.google.com/chromiumos/overlays/chromiumos-overlay/+/1bbf732b3cd306408e2d76e5584a44acd80f7e38/chromeos-base/chromeos-chrome/chromeos-chrome-9999.ebuild#233

So there's no need to give it a smart default.

Bug: 937821
Change-Id: I777e8e7fe398d1a74f755de2061817924f156507
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2161408
Commit-Queue: Ben Pastene <bpastene@chromium.org>
Reviewed-by: Nico Weber <thakis@chromium.org>
Reviewed-by: George Burgess <gbiv@chromium.org>
Cr-Commit-Position: refs/heads/master@{#761735}