[whatwg/fetch] Request's associated element (#912)

[Element Timing](https://wicg.github.io/element-timing) has a concept of a Request's [associated element](https://wicg.github.io/element-timing/#associated-element), which is currently not well defined in terms of Fetch. I'd like to better define that, by being able to pass along the associated element when fetching an image at [updating the image data](https://html.spec.whatwg.org/multipage/images.html#updating-the-image-data).

Any objections to take that approach? Better ways to achieve the same?

/cc @annevk @domenic @npm1


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