I'll take this as well as it's pretty interrelated with the other authentication_service_* migrations

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/4c026001db6b24eac1ea4c90b35c6f7d6ff6e99c

commit 4c026001db6b24eac1ea4c90b35c6f7d6ff6e99c
Author: Sergio Villar Senin <svillar@igalia.com>
Date: Tue Nov 13 20:10:23 2018

Convert authentication_service_unittest.mm to IdentityManager

Partially migrates authentication_service_unitttest.mm to
IdentityManager. In particular it does no longer observ
OAuth2TokenService::Observer but IdentityManager::Observer. Apart from
that a IdentityTestEnvironment was created to wrap all the testing
factories used by the test. The IdentityManager returned by that
object is then used to completely replace the calls to the
SigninManager object.

Bug:  890821 
Change-Id: I0a04a4e8e605546a96606ecb9a3a2338e9060257
Reviewed-on: https://chromium-review.googlesource.com/c/1323552
Commit-Queue: Sergio Villar <svillar@igalia.com>
Reviewed-by: Colin Blundell <blundell@chromium.org>
Reviewed-by: David Roger <droger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#607718}
[modify] https://crrev.com/4c026001db6b24eac1ea4c90b35c6f7d6ff6e99c/ios/chrome/browser/signin/BUILD.gn
[modify] https://crrev.com/4c026001db6b24eac1ea4c90b35c6f7d6ff6e99c/ios/chrome/browser/signin/authentication_service_unittest.mm

Closing

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/e73ecbd3095ffcebb1f52d499230cbb9df6dc77a

commit e73ecbd3095ffcebb1f52d499230cbb9df6dc77a
Author: Sergio Villar Senin <svillar@igalia.com>
Date: Tue Nov 20 09:51:57 2018

Convert iOS AuthenticationService classes to IdentityManager

This is a follow up of crrev.com/c/1335567. The only missing bit
pending to be migrated was some SigninManager API that is now replaced
by the PrimaryAccountMutator API.

Bug:   890817  ,  890818 ,  890819 ,  890820 ,  890820 
Change-Id: I9c746457987038bc810163dbb69dafdba75a74f0
Reviewed-on: https://chromium-review.googlesource.com/c/1341537
Reviewed-by: Sylvain Defresne <sdefresne@chromium.org>
Commit-Queue: Sergio Villar <svillar@igalia.com>
Cr-Commit-Position: refs/heads/master@{#609651}
[modify] https://crrev.com/e73ecbd3095ffcebb1f52d499230cbb9df6dc77a/ios/chrome/browser/signin/authentication_service.h
[modify] https://crrev.com/e73ecbd3095ffcebb1f52d499230cbb9df6dc77a/ios/chrome/browser/signin/authentication_service.mm
[modify] https://crrev.com/e73ecbd3095ffcebb1f52d499230cbb9df6dc77a/ios/chrome/browser/signin/authentication_service_factory.mm
[modify] https://crrev.com/e73ecbd3095ffcebb1f52d499230cbb9df6dc77a/ios/chrome/browser/signin/authentication_service_fake.h
[modify] https://crrev.com/e73ecbd3095ffcebb1f52d499230cbb9df6dc77a/ios/chrome/browser/signin/authentication_service_fake.mm
[modify] https://crrev.com/e73ecbd3095ffcebb1f52d499230cbb9df6dc77a/ios/chrome/browser/signin/authentication_service_unittest.mm

Manually pasted the commit reference as I forgot to mention this bug in the commit message (I duplicated 890820 instead of having 890820 and 890821)