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This fixes a race in GaiaScreenHandler method for setting test user
credentials not to start AddUserFlow (and force GAIA reload) if the auth
extension load has started (in addition to when the gaia frame started
loading) - this was causing issues because the flow would clear up the
current sign-in partition, without stopping the current auth extension
load (and later attempt to authenticate would crash on current signin
storage partition session being empty).
NOTE: This is a wider issue that I plan to look more closely into, but
this change should unblock tests.

BUG=936041,891277

Change-Id: Ie1f76daf0278af59af8b85c4a0fd82b9b0223555
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1560534
Commit-Queue: Alexander Alekseev <alemate@chromium.org>
Auto-Submit: Toni Baržić <tbarzic@chromium.org>
Reviewed-by: Alexander Alekseev <alemate@chromium.org>
Cr-Commit-Position: refs/heads/master@{#649650}