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Tests that the terms of service screen (if required by policy) is the
only screen shown to public session user during login.

To make environment setup for different configurations eaiser, separate
end to end test setup steps from OobeInteractiveUiTest into a separate
mixin.

BUG=none

Change-Id: Ib6580e2b4f63f59c7b262fb0f79982197251724c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1607103Reviewed-by: Alexander Alekseev <alemate@chromium.org>
Commit-Queue: Toni Baržić <tbarzic@chromium.org>
Cr-Commit-Position: refs/heads/master@{#660269}