Pass in Content-Encoding to resource-timing

[Jxck]

These changes are to support addition of content type attribute to Perfomance resource timing. Further details are available at https://github.com/w3c/resource-timing/issues/381

• [x] At least two implementers are interested (and none opposed):
  • Discussed and agreed at Feb 29, 2024 W3C WebPerf call:
• [x] Tests are written and can be reviewed and commented upon at:
  • https://chromium-review.googlesource.com/c/chromium/src/+/5369775
• [x] Implementation bugs are filed:
  • Chromium: https://issues.chromium.org/issues/327941462
  • Gecko: https://bugzilla.mozilla.org/show_bug.cgi?id=1886107
  • WebKit: https://bugs.webkit.org/show_bug.cgi?id=271632
  • Deno (not for CORS changes): …
• [x] MDN issue is filed:
  • https://github.com/mdn/content/issues/32823
• [x] The top of this comment includes a clear commit message to use.

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

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