The Clients API allows service workers to observe the existence of other windows. This API can also be used to infer when windows have been closed. If the window is in bfcache we should not resurrect the window again if script has observed it is gone. Therefore we should evict windows from bfcache if Clients API would have operated on it while its frozen. I had originally planned to do this as part of bug 1293277, but since we don't currently do this now I think it can wait to a follow-up bug. Its also a bit of a corner case and unlikely to have serious interop issues in the short term.
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