Add custodians' obfuscated gaia ids to supervised users' prefs.

The obfuscated GAIA IDs were already being synced, but not
saved to prefs, for access by the SupervisedUserService.

The custodian obfuscated GAIA is needed in order to use the
GAIA Reauth API to reauth a parent of a child/supervised user.

Bug: 957832
Change-Id: If08851ef97a42253dbb45d837cf2a0585a420733
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1597634
Commit-Queue: Danan S <danan@chromium.org>
Reviewed-by: Michael Giuffrida <michaelpg@chromium.org>
Reviewed-by: David Roger <droger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#658192}
5 files changed