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After recent changes, this class no longer introduces any functionality
over SigninManager and hence can be eliminated.

Bug: 796544
Change-Id: Iaea1bb16267dc54808cb4c736e6d3fd4b7edcb09
Reviewed-on: https://chromium-review.googlesource.com/c/1486337Reviewed-by: David Roger <droger@chromium.org>
Commit-Queue: Colin Blundell <blundell@chromium.org>
Cr-Commit-Position: refs/heads/master@{#635531}