Re: [fetch] Introduce convenience property for request's context (#93)

For exposing it to script these cases have come up:

* Revealing to the service worker whether something is a navigation. (I think there might be value for other types too, if you have e.g. transpiling going on for images and you don't want to have to inspect the response before setting up that code.)
* As a convenient way to influence `Accept` and the user agent's notion of "priority" for fetches initiated through `fetch()` and various preloading mechanisms.

And to a lesser extent the idea has been to expose to script the same hooks that CSP, Mixed Content, etc. get so you can implement your own policies in service workers.

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