Re: [whatwg/fetch] Double-keyed HTTP cache (#904)

Rob's question is an interesting one, and I think it's important to be explicit about what we know and what we don't know. 

To the question of "does this change operate a CDN operator who benefits from a shared non-partitioned cache", the answer is clearly **yes**-- the _benefit_ of a shared cache goes away when the cache isn't shared. Reuse of a resource from a cross-site context will no longer be possible when cache partitioning takes effect.

Now, it's also fair to point out that the Chrome team's data suggests that, in total, the benefit of non-partitioned cache *for a particular user* is often small (reducing cache hit rate by ~4%, making the cache something like 10% less effective if the cache hit rate were ~40%.)

However, that end-user impact could look very different to the folks at FontAwesome. I'm haven't seen any data that reveals FontAwesome's current cache hit rate. It could be the case that, on average, each FontAwesome resource is reused on two different sites visited by their average user, in which case the new cache partitioning will _double_ the bytes-on-wire load for the FontAwesome CDNs, even as that individual user themselves would only see a modest % increase in bytes-on-wire (because reused fonts represent only a small percentage of their traffic).

It might be _interesting_ for the Chrome folks to do a study that breaks down the bytes-on-wire cost to the top reused websites (e.g. I predict Google Analytics, Google Fonts, Doubleclick etc are top hitters) so the impact on the servers could be better reasoned about.

From your side, Rob-- does FontAwesome do anything that attempts to measure reuse across sites? E.g. you could conceivably change some percentage of your traffic to use a short lifetime `Vary: Origin` and/or and a [Stale-while-revalidate](https://www.chromestatus.com/feature/5050913014153216) cache-control directive, and watch your logs to get an upper-bound on how much reuse is occurring today.


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