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Use the public API of IdentityManager, PrimaryAccountMutator, to set the
primary account instead of using IdentityManager's internals
(PrimaryAccountManager).

PrimaryAccountMutatorImpl::SetPrimaryAccount calls PrimaryAccountManager::Signin, with added pre-conditions:
  - Preferences allow the signin
  - There is no primary account already set (IsAuthenticated)
  - AccountTrackerService knows about the account, and it has both a
    valid account_id and email.
If those are not met, SetPrimaryAccount does not call PrimaryAccountManager::SignIn and returns false.

In practice, those are already met or were already required:
  - Most of SignIn calls do check the preferences beforehand. Two
    exceptions: Re-authentication cases (SyncAndServicesPreferences, SignInHelper), SigninFragment.
  - PrimaryAccountManager::Signin and
    PrimaryAccountManager::SetAuthenticatedAccountId also verify that.
    If the user is authenticated, the signin is not completed, no
    notifications are sent. A slight behavior change could happen here
    though: a DCHECK verifies that the same user is re-authenticating.
  - PrimaryAccountManager::SignIn also needs AccountTrackerService to be
    properly initialized with the account.

Bug: 987965
Change-Id: I70f30bcf3536a79bb5983333233c59aeb5b8a5a1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1738696
Commit-Queue: Pâris Meuleman <pmeuleman@chromium.org>
Reviewed-by: Sylvain Defresne <sdefresne@chromium.org>
Reviewed-by: Boris Sazonov <bsazonov@chromium.org>
Auto-Submit: Pâris Meuleman <pmeuleman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#685149}