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Previously, ArcSession reported GENERIC_BOOT_FAILURE to its observer,
but it didn't immediately shut down the container. This CL fixes the
issue.

BUG=chromium:737148
TEST=ARC++ still starts

Change-Id: I8dce70a91c521909406594ac325e84765ee0cb7a
Reviewed-on: https://chromium-review.googlesource.com/579167Reviewed-by: Hidehiko Abe <hidehiko@chromium.org>
Commit-Queue: Yusuke Sato <yusukes@chromium.org>
Cr-Commit-Position: refs/heads/master@{#488515}