[ServiceWorker] Support modifying cookies on an intercepted fetch (#837)

There is an important use case around auth and identity where you may want to intercept fetch requests, detect the cookies are out of date, generate new ones (via a fetch to the server) and then send the original request with the new cookies.

Is that possible with SW today? It seems to me that you can't modify the cookie headers (), and it's unclear whether a currently intercepted request will be modified by a `setCookie` call.

@eliaswald @slightlyoff 

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Reply to this email directly or view it on GitHub:
https://github.com/slightlyoff/ServiceWorker/issues/837