I need this one before I can provide a CL for 890796, assigning.

Actually, it's the other way around I just noticed: this can be ported already with the exception of the call to GetMessagesForAvatarSyncError(), that will need to receive a SigninManager until 890796 gets fixed.

I've put up a CL that has a TODO in it to account for that piece of work then, necessary before we can completely remove all trace of SigninManagerFactory:
https://chromium-review.googlesource.com/1261980

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/51d50aaa22fad8fb1f67a81d344321e4631b0e82

commit 51d50aaa22fad8fb1f67a81d344321e4631b0e82
Author: Mario Sanchez Prada <mario@igalia.com>
Date: Mon Oct 08 17:40:47 2018

Migrate AvatarToolbarButton to the IdentityManager

Port all calls to SigninManager::IsAuthenticated() into calls
to IdentityManager::HasPrimartyAccount() instead.

Bug:  890803 
Change-Id: I6b6ca0aeed0dc217c3d44e2d1fdf8215acbc8be6
Reviewed-on: https://chromium-review.googlesource.com/c/1261980
Reviewed-by: Colin Blundell <blundell@chromium.org>
Reviewed-by: David Roger <droger@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Commit-Queue: Mario Sanchez Prada <mario@igalia.com>
Cr-Commit-Position: refs/heads/master@{#597601}
[modify] https://crrev.com/51d50aaa22fad8fb1f67a81d344321e4631b0e82/chrome/browser/ui/views/profiles/avatar_toolbar_button.cc