Modify spec to incorporate storage-access-for-fedcm proposal

[cfredric]

This makes the spec changes for #204.

• At least two implementers are interested (and none opposed):
  • Chrome (proposer)
  • Firefox: positive
  • WebKit: pending
• Tests are written and can be reviewed and commented upon at:
  • Add WPTs for FedCM/SAA autogrants web-platform-tests/wpt#47728
• Implementation bugs are filed:
  • Chromium: https://crbug.com/330574529
  • Gecko: https://bugzilla.mozilla.org/show_bug.cgi?id=1917280
  • WebKit: https://bugs.webkit.org/show_bug.cgi?id=279267

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Preview | Diff

[johannhof]

This looks good overall but happy to take another look since I'm asking for some bigger changes here.

[johannhof]

Looks good, thanks for cleaning up all the same site usage. Merging this given the checklist is complete but happy to make changes based on other editor's feedback later if needed.