Taking a look to this one now...

Uploaded a first CL for review at https://chromium-review.googlesource.com/c/chromium/src/+/1250968

Please let me know what you think, thanks!

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/5942a79fe699bb88b55349aaaaa65d7df26ebdda

commit 5942a79fe699bb88b55349aaaaa65d7df26ebdda
Author: Mario Sanchez Prada <mario@igalia.com>
Date: Mon Oct 08 15:42:47 2018

Add new API: IdentityManager::GetPrimaryAccountId()

This new API will provide access to the account ID of the latest cached
information for the user's primary account, which is conceptually not
the same than getting an AccountInfo with GetPrimaryAccountInfo() and
then accessing the account_id member, since that won't return any ID
when the account has been removed from the AccountTrackerService.

Bug:  887264 
Change-Id: Id87e58fbe86e2a048067d4ede8d9621e2d430cff
Reviewed-on: https://chromium-review.googlesource.com/c/1264162
Commit-Queue: Mario Sanchez Prada <mario@igalia.com>
Reviewed-by: Colin Blundell <blundell@chromium.org>
Cr-Commit-Position: refs/heads/master@{#597567}
[modify] https://crrev.com/5942a79fe699bb88b55349aaaaa65d7df26ebdda/services/identity/public/cpp/identity_manager.cc
[modify] https://crrev.com/5942a79fe699bb88b55349aaaaa65d7df26ebdda/services/identity/public/cpp/identity_manager.h
[modify] https://crrev.com/5942a79fe699bb88b55349aaaaa65d7df26ebdda/services/identity/public/cpp/identity_manager_unittest.cc

(still a second patch pending to land)

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/81eb18725b56153da09f926a3f50ea73aae05d0a

commit 81eb18725b56153da09f926a3f50ea73aae05d0a
Author: Mario Sanchez Prada <mario@igalia.com>
Date: Mon Oct 08 17:43:47 2018

Migrate AdvancedProtectionStatusManager to the IdentityManager

This CL moves AdvancedProtectionStatusManager away from using SiginManager
and related APIs (e.g. SiginManagerBase::Observer), replacing those bits
with calls to the IdentityManager's API instead.

Likewise it moves from fetching tokens using the PO2TS APIs to building
a PrimaryAccessTokenFetcher and relying on it to retrieve the tokens
associated with the primary account.

Bug:  887264 
Change-Id: I80b55190133f05a50412d4a09613176ce24fbeaa
Reviewed-on: https://chromium-review.googlesource.com/c/1250968
Commit-Queue: Mario Sanchez Prada <mario@igalia.com>
Reviewed-by: Jialiu Lin <jialiul@chromium.org>
Cr-Commit-Position: refs/heads/master@{#597602}
[modify] https://crrev.com/81eb18725b56153da09f926a3f50ea73aae05d0a/chrome/browser/safe_browsing/advanced_protection_status_manager.cc
[modify] https://crrev.com/81eb18725b56153da09f926a3f50ea73aae05d0a/chrome/browser/safe_browsing/advanced_protection_status_manager.h
[modify] https://crrev.com/81eb18725b56153da09f926a3f50ea73aae05d0a/chrome/browser/safe_browsing/advanced_protection_status_manager_factory.cc
[modify] https://crrev.com/81eb18725b56153da09f926a3f50ea73aae05d0a/chrome/browser/safe_browsing/advanced_protection_status_manager_unittest.cc

Now it's for real

 Issue 890786  has been merged into this issue.