Update element adoption logic for scoped registry

[ja-y-son]

Fixing #1429 to make sure element is getting the correct scoped custom element registry upon adoption.

• At least two implementers are interested (and none opposed):
  • Webkit
  • Chromium
• Tests are written and can be reviewed and commented upon at:
  • Add test cases for null registry element upon adoption web-platform-tests/wpt#56423
• Implementation bugs are filed:
  • Chromium: https://issues.chromium.org/issues/399124619
  • Gecko: https://bugzilla.mozilla.org/show_bug.cgi?id=1874414
  • WebKit: https://bugs.webkit.org/show_bug.cgi?id=302970
  • Deno (only for aborting and events): …
  • Node.js (only for aborting and events): …
• MDN issue is filed: N/A
• The top of this comment includes a clear commit message to use.

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

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[annevk]

Why is this not using the "effective global custom element registry" strategy discussed? It's not clear to me this is the same or why we need to be looking at document at all.

[ja-y-son]

Updated to use "effective global custom element registry" strategy as discussed. Was trying to see if I could get the point across with another approach but it seems like "effective global custom element registry" is still the better way to go. Lmk if I'm missing anything on this idea.

[annevk]

This looks good to me. We'll need tests next.

For the tests I'd like us to exercise some of the more complicated scenarios with some nodes having been assigned registries and others not, etc.

[ja-y-son]

Just added some tests but I haven't fully verified them yet. Feel free to take a look.