Do not wait for Gaia screen on MgS/ARC Kiosk auto-start.

The check was migrated from old webui waiting for OnSigninScreenReady()
However, OnGaiaScreenReady() might be called a couple of seconds later
then the login screen is actually shown, causing long-long delay for
Mgs/ArcKiosk auto-launch.

Bug: 902315
Test: Manual
Change-Id: I4919ab826bb31075db33d40d54bcb85227d84d54
Reviewed-on: https://chromium-review.googlesource.com/c/1470793
Reviewed-by: Jacob Dufault <jdufault@chromium.org>
Reviewed-by: Alexander Alekseev <alemate@chromium.org>
Commit-Queue: Sergey Poromov <poromov@chromium.org>
Cr-Commit-Position: refs/heads/master@{#632561}
3 files changed