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This CL enables the usage of the Gaia ID as the account ID for the
account manager for profiles that have no accounts on start-up. This
allows us to start using Gaia ID as the account ID on ChromeOS as well.

Using this strategy has very little risk as there should not be any
data that needs to be migrated from email to Gaia ID.

Bug: 1028182

Change-Id: I90cbcf4a9bb8be848276b64d8802e090de428a0b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2491365Reviewed-by: Kush Sinha <sinhak@chromium.org>
Reviewed-by: Sergey Poromov <poromov@chromium.org>
Reviewed-by: Marc Treib <treib@chromium.org>
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Reviewed-by: Boris Sazonov <bsazonov@chromium.org>
Auto-Submit: Mihai Sardarescu <msarda@chromium.org>
Commit-Queue: Xiyuan Xia <xiyuan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#828215}