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This CL adds IdentityTestEnvironment APIs to interact with the refresh
token state of secondary accounts; these APIs will be needed in an
upcoming CL. In this CL we also change all such test APIs that return
the account ID of a newly-set account to instead return the AccountInfo;
for the upcoming use case, this will be convenient.

Bug: 798699
Change-Id: I9deceae9fb016147fce6206c263acf638d21ef6e
Reviewed-on: https://chromium-review.googlesource.com/1107984
Commit-Queue: Colin Blundell <blundell@chromium.org>
Reviewed-by: Marc Treib <treib@chromium.org>
Cr-Commit-Position: refs/heads/master@{#569201}