CL in progress here:
https://chromium-review.googlesource.com/c/chromium/src/+/1257834

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/a70958a47152e3fcf66fb3854e9d2fd39a569c98

commit a70958a47152e3fcf66fb3854e9d2fd39a569c98
Author: Mario Sanchez Prada <mario@igalia.com>
Date: Thu Oct 04 13:54:39 2018

Convert SigninObserverBridge to IdentityManager::Observer

Migrated to IdentityManager::Observer and implemented the corresponding
virtual functions from there.

Bug:  887465 
Cq-Include-Trybots: luci.chromium.try:ios-simulator-cronet;luci.chromium.try:ios-simulator-full-configs
Change-Id: I9436fe11d90b7593d05a0bf97ab11134d12105b7
Reviewed-on: https://chromium-review.googlesource.com/c/1257834
Commit-Queue: Mario Sanchez Prada <mario@igalia.com>
Reviewed-by: Sylvain Defresne <sdefresne@chromium.org>
Cr-Commit-Position: refs/heads/master@{#596651}
[modify] https://crrev.com/a70958a47152e3fcf66fb3854e9d2fd39a569c98/ios/chrome/browser/ui/settings/BUILD.gn
[modify] https://crrev.com/a70958a47152e3fcf66fb3854e9d2fd39a569c98/ios/chrome/browser/ui/settings/settings_collection_view_controller.mm