Stop infinite restarting

Dumb logic to stop infinite restarting of the child job (chrome).
Now, if we ever try to start the child twice in one second, we decide
to (instead) tear everything down and exit the session manager.  This
should lead to whole UI getting restarted.

Eventually, we'd like to do something smarter where we try N times in M seconds, or do an exponential backoff or something.  but for now, this keeps users from getting into an infinite loop of starting/crashing chrome.

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