Add unittests for ArcSessionImpl.

BUG=739118
TEST=Ran bots.

Change-Id: Ic4c81c914974b8d860cc65bcbc6e5cc67695f3fe
Reviewed-on: https://chromium-review.googlesource.com/768618


Reviewed-by: Luis Hector Chavez <lhchavez@chromium.org>
Reviewed-by: Dan Erat <derat@chromium.org>
Reviewed-by: Yusuke Sato <yusukes@chromium.org>
Commit-Queue: Hidehiko Abe <hidehiko@chromium.org>
Cr-Commit-Position: refs/heads/master@{#518299}