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This CL migrates ArcUserSessionService.

BUG=672829
TEST=Ran try.

Change-Id: Icdd45e1f52b2372bd3613266ed96fa463c801a9a
Reviewed-on: https://chromium-review.googlesource.com/572474
Commit-Queue: Hidehiko Abe <hidehiko@chromium.org>
Reviewed-by: Yusuke Sato (in China Mon-Thurs, may be offline) <yusukes@chromium.org>
Cr-Commit-Position: refs/heads/master@{#487335}