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This allows arc::ArcSessionManager to restart the VM when needed.

BUG=b:135556154, b:141966414
TEST=kill off ARCVM's crosvm, verify the observer detects the event.

Change-Id: Iba71de782db078b1b89d196ef06c749ebd17f260
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1841250Reviewed-by: Hidehiko Abe <hidehiko@chromium.org>
Commit-Queue: Yusuke Sato <yusukes@chromium.org>
Auto-Submit: Yusuke Sato <yusukes@chromium.org>
Cr-Commit-Position: refs/heads/master@{#704091}