[whatwg/fetch] Revamp body's total bytes as length (#1184)

Together with #1183 this solves the Fetch side of #604. Changes:

* No longer set a length for network responses. Callers will have to parse Content-Length themselves.
* Make extract set length so requests will set Content-Length correctly.

XMLHttpRequest PR: TODO.

Tests: https://github.com/web-platform-tests/wpt/pull/27837.


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You can view, comment on, or merge this pull request online at:

  https://github.com/whatwg/fetch/pull/1184


-- Commit Summary --

  * Revamp body's total bytes as length

-- File Changes --

    M fetch.bs (123)

-- Patch Links --

https://github.com/whatwg/fetch/pull/1184.patch

https://github.com/whatwg/fetch/pull/1184.diff


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