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As noted in b/143229899, a stale VmStopped signal can be sent to
Chrome if StopAllVms() call sent by session_manager takes a long
time to finish. Without this CL, such a signal could confuse the
VM adapter.

BUG=b:143229899
TEST=new tests pass

Change-Id: I72b918aebfc8b1218d25c931ce61bde728614a32
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1877603
Auto-Submit: Yusuke Sato <yusukes@chromium.org>
Commit-Queue: Yusuke Sato <yusukes@chromium.org>
Reviewed-by: Hidehiko Abe <hidehiko@chromium.org>
Cr-Commit-Position: refs/heads/master@{#709318}