I've checked flipping the default value and it does not affect any of the tests that have been running it with critical=False (other than making these tests pass if ran directly after provision)

autoupdate_EOL
autoupdate_OmahaResponse.url_switch
autoupdate_ForcedOOBEUpdate.cellular.*

The following revision refers to this bug:
  https://chromium.googlesource.com/chromiumos/third_party/autotest/+/8f43ada3019bf537ca1f0f9a7b44ad6f8d5f57fb

commit 8f43ada3019bf537ca1f0f9a7b44ad6f8d5f57fb
Author: David Haddock <dhaddock@chromium.org>
Date: Tue Apr 10 23:29:18 2018

Make nano_omaha_devserver default to returing critical updates.

This ensures that we can run autoupdate tests right after provision.
Otherwise I sometimes get this error:
"Ignoring non-critical Omaha updates until OOBE is done".

The following tests use nano_omaha_devserver:
autoupdate_EOL
autoupdate_OmahaResponse.url_switch
autoupdate_ForcedOOBEUpdate.cellular.*

Each of these tests should pass whether the critical update is True or False.
However, sometimes when the test is kicked off directly after provision,
update_engine complains that it is ignoring non critical updates until OOBE is
done.

BUG= chromium:830951 
TEST=autoupdate tests that rely on nano_omaha_devserver.

Change-Id: I4a36c968207dd0818a0a81f87148a6c23078afbd
Reviewed-on: https://chromium-review.googlesource.com/1003531
Commit-Ready: David Haddock <dhaddock@chromium.org>
Tested-by: David Haddock <dhaddock@chromium.org>
Reviewed-by: Amin Hassani <ahassani@chromium.org>

[modify] https://crrev.com/8f43ada3019bf537ca1f0f9a7b44ad6f8d5f57fb/client/site_tests/autoupdate_StartOOBEUpdate/autoupdate_StartOOBEUpdate.py
[modify] https://crrev.com/8f43ada3019bf537ca1f0f9a7b44ad6f8d5f57fb/client/cros/update_engine/nano_omaha_devserver.py