[whatwg/fetch] Add support for Content-DPR response header (#971)

This WIP PR, in conjunction with an (incoming) PR to HTML, adds support for the `Content-DPR` response header on image responses ([see explainer here](https://github.com/eeeps/content-dpr-explainer)).
You can view, comment on, or merge this pull request online at:

  https://github.com/whatwg/fetch/pull/971

-- Commit Summary --

  * Copy/paste @yoav's text

-- File Changes --

    M fetch.bs (10)

-- Patch Links --

https://github.com/whatwg/fetch/pull/971.patch
https://github.com/whatwg/fetch/pull/971.diff

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