[fetch] Deal with nested browsing contexts (#120)

Per discussion with @mikewest it seem best add "target browsing context". It is only relevant during navigation so need to be able to be null.

We might want to rename "client" to "requesting client" to make it more clear which of the various globals it is that is involved with navigating, but I'm not sure the churn cost to other specifications makes that worth it.

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Reply to this email directly or view it on GitHub:
https://github.com/whatwg/fetch/issues/120