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This is a reland of 144e9e93

Making user session manager open a browser on startup fixed this
for me locally (without it, chrome shutdown would start just after
login screen UI goes away).

To be safe added a waiter to make sure the accept button is there when
the test attempts clicking it.

TBR=alemate@chromium.org

Original change's description:
> Test oobe screens for public session login
>
> 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/+/1607103
> Reviewed-by: Alexander Alekseev <alemate@chromium.org>
> Commit-Queue: Toni Baržić <tbarzic@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#660269}

Bug: none
Change-Id: Ib3e9a7adf3486122ae3ddb5c8c2cd4a8e87915ca
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1614151Reviewed-by: Toni Baržić <tbarzic@chromium.org>
Commit-Queue: Toni Baržić <tbarzic@chromium.org>
Cr-Commit-Position: refs/heads/master@{#660656}