[fetch] Surface requests for multi-request fetches (redirects, preflights, etc) (#75)

Some fetches incur multiple requests before they are fulfilled - e.g. redirects, permission preflights, resumed upload/downloads, etc. Today, these are hidden and not observable (although sometimes you can infer that a redirect happened based on type of response you get), and we need a mechanism to make them observable, such that they can be measured and accounted for -- this is a common [issue and request filed against Nav and Resource timing](https://github.com/w3c/resource-timing/issues/21).

We already have [Timing-Allow-Origin](http://www.w3.org/TR/resource-timing/#timing-allow-origin) to signal that timing data is allowed to be shared with particular origins. Perhaps we need a similar header to "reveal" the intermediate requests? Or, perhaps we can extend TAO to allow this?

1. As of today TAO is a no-op for same-origin requests. We could extend it to act as a signal for same- and third-party origins that intermediate requests are allowed to be surfaced.
2. We could add a separate header with similar definition and semantics as TAO. This would allow behavior where you can allow intermediate requests to be surfaced but opt out from providing detailed timing data. This offers more granular control but at the cost of yet another response header (overhead).

Thoughts? 

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Reply to this email directly or view it on GitHub:
https://github.com/whatwg/fetch/issues/75