Bumping this up to P1 as it's blocking the conversion of the SigninTracker unittest away from direct usage of PO2TS/SigninManager and is thus blocking the completion of the current milestone of removing all direct usage of those classes. Camille, would you be interested in taking this on as part of the SigninTracker work?

We can call the API OnGaiaAccountAddedToCookie() for consistency with the sibling observer API OnGaiaAccountsInCookieUpdated().

Antonio is going to take this on as it's blocking a conversion that he's looking to do. CC'ing Lowell, who will be managing the GaiaCookieManagerService conversion over all Real Soon Now (TM). Antonio, when you send out the CL, could you send it to both me and Lowell for review? Thanks!

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/053d9abfe3ccad58e14052dd5fc2b5a5dd7906cd

commit 053d9abfe3ccad58e14052dd5fc2b5a5dd7906cd
Author: Antonio Gomes <tonikitoo@igalia.com>
Date: Fri Dec 14 17:01:01 2018

Develop IdentityManager::Observer equivalent to GCMS::Observer::OnAddAccountToCookieCompleted

CL adds an equivalent hook to
GaiaCookieManagerService::Observer::OnAddAccountToCookieCompleted
in IdentityManager::Observer.

This is part of the effort to allow GCMS::Observer be replaced by
IdentityManager::Observer. No functionality changes expected.

The unittests aim at ensuring the hook is called through
Observers of IdentityManager in case of successful or failing session
merges.

BUG= 903718 

Change-Id: Id08c4515171f3c21e134c2c129593e8977bd7e11
Reviewed-on: https://chromium-review.googlesource.com/c/1376529
Commit-Queue: Antonio Gomes <tonikitoo@igalia.com>
Reviewed-by: Colin Blundell <blundell@chromium.org>
Cr-Commit-Position: refs/heads/master@{#616713}
[modify] https://crrev.com/053d9abfe3ccad58e14052dd5fc2b5a5dd7906cd/services/identity/public/cpp/identity_manager.cc
[modify] https://crrev.com/053d9abfe3ccad58e14052dd5fc2b5a5dd7906cd/services/identity/public/cpp/identity_manager.h
[modify] https://crrev.com/053d9abfe3ccad58e14052dd5fc2b5a5dd7906cd/services/identity/public/cpp/identity_manager_unittest.cc