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

@annevk what exactly is "contents of a redirect"? The URL during that hop? 3XX's don't carry a response payload. Also, note that for the purpose of Nav and Resource Timing we're not after payloads, headers, or response codes. Rather, we want to surface that a redirect hop / preflight occurred, plus some timing metrics around it. 

Actually, thinking out loud... Are there any issues with simply surfacing the fact that a redirect happened without disclosing any other information? For example, say I fetch A, which bounces me to B and that reroutes me to C. If I query via Resource Timing (`performance.getEntriesByName("A")`), I'd see:

```
[
PerformanceResourceTiming{ name: A }, 
PerformanceResourceTiming{ name: A },
PerformanceResourceTiming{ name: A, duration: X, domainLookupStart: ..., ... }
]
```

* We surface redirects as "empty" ResourceTiming events.
* If redirect URL provides TAO header, we could also surface timing metrics for that hop.
* All events have same name (resolved URL that was originally requested); optionally we can figure out if/how we could allow surfacing the hop URL?

Lots of details to think through here, but to start, thoughts on disclosing the request without providing any detailed information about it?

/cc @mikewest

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