[whatwg/fetch] Expose PerformanceResourceTiming in Response Objects (#491)

## Problem

Currently the process of linking a fetch request to a specific resource timing object is imperfect. If there are two fetch api requests for the same resource that happen within a short time it's impossible to tell exactly which fetch call initiated which request, leading to some errors in performance logging. Orthogonally, if all a developer cares about is the perf result for a single fetch, having to use the whole performance api could be cumbersome.

## Solution?

The ideal solution here (imo) is for there to be a way to get a resource timing object for a specific response. This could be accomplished by adding a method to response objects that give you a promise which will eventually resolve to the PerformanceResourceTiming object once it's ready.

Thoughs?

*Note: Since I think this should be a change to the fetch api I filed the issue here as opposed to the performance timeline repo. Happy to move it tough if we think it's better suited there.*

(cc, @igrigorik, @spanicker for their thoughts)


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