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The main goal is to avoid session start delay while chrome is
attempting to fetch fresh child user's policy. The user session manager
implements fetch timeout (so tests pass without this), but waiting for
it adds some 10s to test login.

To fix this:
*   have login manager mixin mark child users as requiring policy
*   add support for configuring policy test server (in addition to
    session manager cache) to user policy mixin
*   add utility method to fake gaia to setup child user credentials
    (needed as policy fetch will be blocked on getting policy
     auth token)

Bug: None

Change-Id: I5a2e65a4cbf1e9ce192fb0ac750a1661f895e462
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1602153Reviewed-by: Alexander Alekseev <alemate@chromium.org>
Reviewed-by: Denis Kuznetsov <antrim@chromium.org>
Reviewed-by: Aga Wronska <agawronska@chromium.org>
Commit-Queue: Toni Baržić <tbarzic@chromium.org>
Cr-Commit-Position: refs/heads/master@{#659256}