After further discussion, we're preserving the invariant by (a) scoping the IdentityManager changes to *only* the account ID used for supervised users and (b) populating the AccountInfo's account ID, gaia ID, and email in this case (the account ID and email coming from the constant used for supervised users, while the gaia ID is a dummy). So the bug here then becomes to remove this special-case code.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/cfc450eaea093816f8598e95ac5b3e50bf2fc3f0

commit cfc450eaea093816f8598e95ac5b3e50bf2fc3f0
Author: Colin Blundell <blundell@chromium.org>
Date: Tue Jul 10 16:52:40 2018

IdentityManager: Handle corner case for supervised users

In IdentityManager, we have been maintaining the invariant that
AccountTrackerService knows of all accounts that
ProfileOAuth2TokenService knows of. In ordinary usage, this invariant is
true (as far as we know). However, in the context of supervised users,
it turns out that it is not true. SupervisedUserService calls
PO2TS::LoadCredentials() and PO2TS::UpdateCredentials() without
interacting with AccountTrackerService whatsoever (e.g.,
https://cs.chromium.org/chromium/src/chrome/browser/supervised_user/supervised_user_service.cc?sq=package:chromium&g=0&l=404).

Changing SupervisedUserService to interact with AccountTrackerService
seems infeasible as we have no way to test those changes and it's
non-obvious what the production impact would be (e.g., that pseudo
account showing up in various UI surfaces, being persisted to disk by
AccountTrackerService, ...).

To handle this case without weakening IdentityManager's invariant, this
CL populates AccountInfo as necessary when dealing with the specific
account ID used for supervised users. This change is concretely
necessary to unblock the landing of necessary changes that tickle
these invariants
(https://chromium-review.googlesource.com/c/chromium/src/+/1126861);
that tickling is how these facts were uncovered.

Bug: 860492
Change-Id: I69e67e900f62e92577a52d628f96b1ae0d9a89e3
Reviewed-on: https://chromium-review.googlesource.com/1127050
Commit-Queue: Colin Blundell <blundell@chromium.org>
Reviewed-by: Marc Treib <treib@chromium.org>
Reviewed-by: Mihai Sardarescu <msarda@chromium.org>
Cr-Commit-Position: refs/heads/master@{#573773}
[modify] https://crrev.com/cfc450eaea093816f8598e95ac5b3e50bf2fc3f0/chrome/browser/supervised_user/supervised_user_constants.cc
[modify] https://crrev.com/cfc450eaea093816f8598e95ac5b3e50bf2fc3f0/services/identity/public/cpp/identity_manager.cc