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When launching (online enrolled) demo sessions with no connectivity, ARC
provisioning for the demo mode public account will timeout (it will
fail to get the account auth token), and restart the public session.

This CL adds a fallback for demo session when the auth token for the
demo account cannot be fetched - it continues ARC provisioning with
offline demo mode provisioning.

BUG=875504
TEST=Start demo session with no network - ARC successfully provisions,
  and the session is not forcibly restarted

Change-Id: I76fe36cf939a5ac8d34b89f821fc142d2618d7d4
Reviewed-on: https://chromium-review.googlesource.com/1180415
Commit-Queue: Toni Baržić <tbarzic@chromium.org>
Reviewed-by: Yusuke Sato <yusukes@chromium.org>
Cr-Commit-Position: refs/heads/master@{#585308}