Further updates from discussion with sdefresne@:

- SigninManagerBase::IsSigninAllowed() is used from within SigninManager, which complicates the idea of moving it to be entirely a client-side concept.
- SigninManager also listens for the kSigninAllowed pref changing its value in order to sign out the user immediately if it flips to not allowed.

This functionality might need to end up part of some distinct privileged interface for interacting with IdentityManager for signin/signout-related use cases.

This concept might just go away (see blocking bug).

Discussion with msarda offline lead to the following decision:

1. the concept will not go away any time soon (or probably ever)
2. the preference needs to live in the embedder (to allow proper control by policy)
3. SigningManager should expose SetSigninAllowed/IsSigninAllowed not backed by pref
4. embedder should call SetSigninAllowed in response to pref changes

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/582d3964d03bdd1ab54f5ce02adc6adfb7053b9e

commit 582d3964d03bdd1ab54f5ce02adc6adfb7053b9e
Author: Sylvain Defresne <sdefresne@chromium.org>
Date: Thu Nov 15 16:22:27 2018

Add implementation for PrimaryAccountMutatorImpl [1/N]

Add implementation and test for PrimaryAccountMutatorImpl
methods {Set,Is}SettingPrimaryAccountAllowed.

Bug: 889902,  806778 
Change-Id: Ibd75f2b3a988b4c403eab3b682a157e31e4b2e18
Reviewed-on: https://chromium-review.googlesource.com/c/1335937
Reviewed-by: Colin Blundell <blundell@chromium.org>
Reviewed-by: Mihai Sardarescu <msarda@chromium.org>
Commit-Queue: Sylvain Defresne <sdefresne@chromium.org>
Cr-Commit-Position: refs/heads/master@{#608390}
[modify] https://crrev.com/582d3964d03bdd1ab54f5ce02adc6adfb7053b9e/components/signin/core/browser/signin_manager.cc
[modify] https://crrev.com/582d3964d03bdd1ab54f5ce02adc6adfb7053b9e/components/signin/core/browser/signin_manager.h
[modify] https://crrev.com/582d3964d03bdd1ab54f5ce02adc6adfb7053b9e/components/signin/core/browser/signin_manager_base.h
[modify] https://crrev.com/582d3964d03bdd1ab54f5ce02adc6adfb7053b9e/services/identity/BUILD.gn
[modify] https://crrev.com/582d3964d03bdd1ab54f5ce02adc6adfb7053b9e/services/identity/public/cpp/BUILD.gn
[modify] https://crrev.com/582d3964d03bdd1ab54f5ce02adc6adfb7053b9e/services/identity/public/cpp/primary_account_mutator_impl.cc
[add] https://crrev.com/582d3964d03bdd1ab54f5ce02adc6adfb7053b9e/services/identity/public/cpp/primary_account_mutator_unittest.cc

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/7b32e1d79f6e38b8824c3cd47f768632680251ce

commit 7b32e1d79f6e38b8824c3cd47f768632680251ce
Author: Sylvain Defresne <sdefresne@chromium.org>
Date: Fri Nov 16 12:03:51 2018

Add implementation for PrimaryAccountMutatorImpl [2/N]

Add implementation and test for PrimaryAccountMutatorImpl
method SetPrimaryAccount. The method is the new API that
will replace SigninManager::OnExternalSigninCompleted().

Bug: 889902,  806778 
Change-Id: I0ece2927c24506644a1087a7ed10df4578d592e3
Reviewed-on: https://chromium-review.googlesource.com/c/1335601
Commit-Queue: Sylvain Defresne <sdefresne@chromium.org>
Reviewed-by: Colin Blundell <blundell@chromium.org>
Cr-Commit-Position: refs/heads/master@{#608735}
[modify] https://crrev.com/7b32e1d79f6e38b8824c3cd47f768632680251ce/services/identity/public/cpp/primary_account_mutator.h
[modify] https://crrev.com/7b32e1d79f6e38b8824c3cd47f768632680251ce/services/identity/public/cpp/primary_account_mutator_impl.cc
[modify] https://crrev.com/7b32e1d79f6e38b8824c3cd47f768632680251ce/services/identity/public/cpp/primary_account_mutator_unittest.cc