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This ends up sending two D-Bus messages to session_manager, and
it is guaranteed that the message for starting ARC instance is
always sent first. This is because Chrome (with crrev.com/488879)
sends the D-Bus method directly in StartArc() without any thread
hops.

BUG=b:64327302
TEST=try, manually went through ARC opt-in

Change-Id: I97c4779b2ed5827b46559f9f7571a1090b4ced68
Reviewed-on: https://chromium-review.googlesource.com/601768
Commit-Queue: Yusuke Sato <yusukes@chromium.org>
Reviewed-by: Hidehiko Abe <hidehiko@chromium.org>
Cr-Commit-Position: refs/heads/master@{#491967}