[whatwg/fetch] Allow .blob() to return disk-backed resource (#569)

It seems that both Chrome & Firefox point to an item on disk when you do:

```js
const cache = await caches.open('test');
const response = await cache.match('/really-large-resource');
const blob = await response.blob();
```

Test: https://cache-memory-test.glitch.me/

This is pretty cool, as you can get a slice of the data without reading it all into memory. However, it seems to go against https://fetch.spec.whatwg.org/#concept-body-consume-body which requires reading the whole response.

We should spec the Chrome/Firefox behaviour, possibly by flagging requests/responses whose body is on disk.

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