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Partially migrates authentication_service_unitttest.mm to
IdentityManager. In particular it does no longer observ
OAuth2TokenService::Observer but IdentityManager::Observer. Apart from
that a IdentityTestEnvironment was created to wrap all the testing
factories used by the test. The IdentityManager returned by that
object is then used to completely replace the calls to the
SigninManager object.

Bug: 890821
Change-Id: I0a04a4e8e605546a96606ecb9a3a2338e9060257
Reviewed-on: https://chromium-review.googlesource.com/c/1323552
Commit-Queue: Sergio Villar <svillar@igalia.com>
Reviewed-by: Colin Blundell <blundell@chromium.org>
Reviewed-by: David Roger <droger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#607718}