Re: [fetch] Cache mode: fromCache (#40)

FWIW - I think this is a bit different than `only-if-cached`. The problem there is that it makes it possible to probe the cache without any corresponding traffic being emitted to the server, making it a passive attack. 

Exposing `fromCache` doesn't do that; if someone tries to use it for probing, it will still emit traffic. 

What it does do is *slightly* improve confidence as opposed to cache timing attacks (but they're pretty conclusive anyway, AIUI).

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