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Identity API tests: Move from SigninManager to IdentityManager

This was fun because these tests used account_id, gaia_id, and email
more or less interchangably, which doesn't work well with
IdentityManager (or with sanity). This CL cleans this up and clearly
differentiates the three.

Bug: 825190
Change-Id: Ibdebb6d295f84ef15dd2864819c98fb1274ab63c
Reviewed-on: https://chromium-review.googlesource.com/1010423Reviewed-by: Mihai Sardarescu <msarda@chromium.org>
Commit-Queue: Marc Treib <treib@chromium.org>
Cr-Commit-Position: refs/heads/master@{#551288}