[fetch] Add hooks for foreign fetch (#223)

slightlyoff/ServiceWorker#751 adds the service worker bits of foreign fetch to the foreign fetch spec. To actually make this work fetch needs to be modified to call "Handle Foreign Fetch" at the appropriate time while fetching (probably between step 3 and 4 of HTTP Fetch).

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Reply to this email directly or view it on GitHub:
https://github.com/whatwg/fetch/issues/223