[whatwg/xhr] Use extract a length from Fetch (#317)

The only information browsers use for progress events for responses is the Content-Length header.

Align on that and only use body's length concept for uploads.

Tests: web-platform-tests/wpt#27837.

Fetch PRs: whatwg/fetch#1183 & whatwg/fetch#1184.

- [ ] At least two implementers are interested (and none opposed):
   * …
   * …
- [ ] [Tests](https://github.com/web-platform-tests/wpt) are written and can be reviewed and commented upon at:
   * …
- [ ] [Implementation bugs](https://github.com/whatwg/meta/blob/main/MAINTAINERS.md#handling-pull-requests) are filed:
   * Chrome: …
   * Firefox: …
   * Safari: …

(See [WHATWG Working Mode: Changes](https://whatwg.org/working-mode#changes) for more details.)


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<a href="https://whatpr.org/xhr/317.html" title="Last updated on Mar 2, 2021, 4:39 PM UTC (e93c89d)">Preview</a> | <a href="https://whatpr.org/xhr/317/26f974f...e93c89d.html" title="Last updated on Mar 2, 2021, 4:39 PM UTC (e93c89d)">Diff</a>
You can view, comment on, or merge this pull request online at:

  https://github.com/whatwg/xhr/pull/317


-- Commit Summary --

  * Handle errors in response bodies
  * Use extract a length from Fetch

-- File Changes --

    M xhr.bs (101)

-- Patch Links --

https://github.com/whatwg/xhr/pull/317.patch

https://github.com/whatwg/xhr/pull/317.diff


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