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This is strictly unnecessary because either vm_concierge.conf job
(in case of user session termination) or session_manager (in case
of browser exit) stops all VMs including ARCVM.

BUG=b:143569709
TEST=arc.Boot
TEST=sign in, sign out, verify StopVm for ARCVM is not called
TEST=sign in, change chrome://flags and restart the browser,
 verify StopVm is not called

Change-Id: I4c3a410d76f43da7a9743d692380b5c86962479e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1889299Reviewed-by: Hidehiko Abe <hidehiko@chromium.org>
Commit-Queue: Yusuke Sato <yusukes@chromium.org>
Cr-Commit-Position: refs/heads/master@{#711275}