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The old SigninManager::IsSignoutProhibited() had a few smells
1. callers of SM::SignOut() were expected to always call SignOut()
   even when prohibited. This allowed the *chrome* client to do special
   cleanup.
2. ProhibitSignout() was really meant to narrowly prohibit user-triggered
   sign-out for scenarios like cloud-managed accounts. But the prohibition
   was applied broadly for all SignOut() calls, leading to weird states
   when sign-out was triggered for internal reasons (e.g. cloud policy
   changes).

These issues came up in planning an analogous API for the new
IdentityManager. This CL implements the proposed fixes
https://docs.google.com/document/d/14esaufZT0fr258zL5I69y7YBBc24XsxtDU0B7wpWxBo/edit#bookmark=id.t69fnqpy377k

Namely:
- Prohibiting sign-out is now up to the SigninClient. SignOut() signals
  the source of the sign-out (user-driven vs internal reasons) and the client
  runs a callback indicating whether the sign-out may continue.
- signin_util::SetUserSignoutAllowedForProfile() chrome/ code
  allow/block user-driven sign-outs.
- Other implementers of SigninClient never block sign-out and are unchanged.

NOTE: For now, signin_util::SetUserSignoutAllowedForProfile() actually
prohibits most non-user-driven signouts to avoid changing sign-out behavior in this
large plumbing CL. This will change in a follow up CL where non-user-driven
sign-outs are always permitted.
https://chromium-review.googlesource.com/c/chromium/src/+/1175797/


Bug: 866518
Cq-Include-Trybots: luci.chromium.try:ios-simulator-cronet;luci.chromium.try:ios-simulator-full-configs
Change-Id: I4758706d796f953cbb68ff1b4a0c8d2985d12fb3
Reviewed-on: https://chromium-review.googlesource.com/1162173
Commit-Queue: Chrome Cunningham <chcunningham@chromium.org>
Reviewed-by: Mihai Sardarescu <msarda@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Reviewed-by: Colin Blundell <blundell@chromium.org>
Reviewed-by: Boris Sazonov <bsazonov@chromium.org>
Cr-Commit-Position: refs/heads/master@{#595214}