[fetch] Track request originator. (#45)

As discussed a bit in https://github.com/mikewest/internetdrafts/commit/d2e4cd80fc9417a4d4d2d74e7d9be2781454c972, the `First-Party-Only` cookie attribute requires tracking the originator of a request across navigations. That doesn't seem possible at the moment via Fetch's current definition.

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Reply to this email directly or view it on GitHub:
https://github.com/whatwg/fetch/issues/45