Use extract a length from Fetch

[annevk]

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.

• [x] At least two implementers are interested (and none opposed):
  • See https://github.com/whatwg/fetch/pull/1184.
• [x] Tests are written and can be reviewed and commented upon at:
  • See above.
• [x] Implementation bugs are filed:
  • See https://github.com/whatwg/fetch/pull/1184.

(See WHATWG Working Mode: Changes for more details.)

---

Preview | Diff

[annevk]

@yutakahirano could you please review this? It will successfully build as soon as the Fetch change is indexed, but the Preview and Diff links are working already so I think reviewing should be possible. Thanks!

[annevk]

Thanks @yutakahirano, should be good now.