Re: [ServiceWorker] Define some way for foreign fetch to decide on opaqueness of responses (#841)

But you could force CORS responses to be at least as protective as the network response. So if you receive an opaque response you could not make it more visible at all and if you receive a _partial_ opaque response (say, exposing just some headers) you could reduce this set but never extending it. 

This way:
```js
self.onforeignfetch = evt => {
    evt.respondWith(self.fetch(evt.request));
};
```
Acts the same way as if it were no service worker.

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