Add reference to MIX in main fetch

[carlosjoan91]

• At least two implementers are interested (and none opposed):
  • This was proposed originally as part of changes to MIX, which have since been implemented by Chrome and are in the process of being implemented by Firefox. Additionally Safari supported the MIX changes.
• Tests are written and can be reviewed and commented upon at:
  • Tests exist under https://github.com/web-platform-tests/wpt/tree/master/mixed-content/tentative/autoupgrades.
• Implementation bugs are filed:
  • Chromium: This behavior is already launched in Chrome
  • Gecko: Unclear if bug exists yet, but it's planned for launching in Nightly soon (see discussion in Fetch Metadata test expectations in light of Mixed Content upgrading for image, audio, video web-platform-tests/wpt#37080)
• MDN issue is filed: Unclear if this necessary for this change since it doesn't add any new features.

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

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Preview | Diff

[carlosjoan91]

@annevk: I was intending to send this to review to you, but I can't seem to edit reviewers, am I missing a pre-contribution step in the fetch repo?

[annevk]

Can you create a PR so the tests are no longer tentative?

Note that at some point we might have to rework this integration if we start doing URL changes through internal redirects: #1426. I think for now it's okay to proceed as-is though.

[carlosjoan91]

Thanks for taking a look, I've now created web-platform-tests/wpt#37633 moving the autoupgrade tests out of tentative.

[annevk]

Thanks @carlosjoan91 for tackling this.