Expose a way to link a call to fetch to an individual PerformanceResourceTiming object

(copy and pasting from https://github.com/whatwg/fetch/issues/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?

Start with the copied context from WHATWG Fetch issue #491 and this issue's proposal; no implementation files or tests are named. Done would require an agreed specification change that links an individual fetch response to its PerformanceResourceTiming object, followed by identifying the affected Resource Timing spec sections and validation work.