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The desktop re-authentication flow does not update the sign-in manager.
Here are the explinations why:
* SigninManager::StartSigninWithRefreshToken dchecks that the profile
is not authenticated (see https://cs.chromium.org/chromium/src/components/signin/core/browser/signin_manager.cc?rcl=a2b0b2c00ad3b7c02b57fe86b9feba1a4c5221fd&l=105)
* InlineSigninHandler simply updates the refresh tokens in case of a
reauth (see https://cs.chromium.org/chromium/src/chrome/browser/ui/webui/signin/inline_login_handler_impl.cc?rcl=a2b0b2c00ad3b7c02b57fe86b9feba1a4c5221fd&l=246 )

The mobile re-auth flow calls SigninManager::OnExternalSignin which in
some cases may lead to a unexpected call to GoogleSigninSucceeded.
On iOS this flow does not exists as the iSL always removes invalid
accounts.
On Android, there is a possibility for the SigninManager::OnExternalSignin
to be called in a reauth flow, however the Java code ignores
GoogleSigninSucceeded notifications (see
https://cs.chromium.org/chromium/src/chrome/browser/android/signin/signin_manager_android.cc?rcl=a2b0b2c00ad3b7c02b57fe86b9feba1a4c5221fd&l=347)

This CL makes it explicit that:
* SigninManager does not send GoogleSigninSucceeded notifications on
reauth
* Adds a DCHECK when the authenticated account ID is changed.

TBR=mpearson@chromium.org

Bug: 733226
Change-Id: I85cc4497a0d099447a7d8a46b435e93f39d8d4c7
Reviewed-on: https://chromium-review.googlesource.com/579432
Commit-Queue: Mihai Sardarescu <msarda@chromium.org>
Reviewed-by: David Roger <droger@chromium.org>
Reviewed-by: Roger Tawa <rogerta@chromium.org>
Reviewed-by: Pavel Yatsuk <pavely@chromium.org>
Cr-Commit-Position: refs/heads/master@{#491983}