Ensure CustomElementRegistry.initialize has CEReaction attribute

[ja-y-son]

Found this issue while implementing #11858. Looking at webkit's implementation and the expected behavior in WPT, I assume it's an oversight during the spec change.

• At least two implementers are interested (and none opposed):
  • Webkit
  • Chromium
• Tests are written and can be reviewed and commented upon at:
  • Add a test for CustomElementRegistry.initialize upgrading custom elements per https://github.com/whatwg/html/pull/11913. web-platform-tests/wpt#56034
• 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=302534
• Corresponding HTML AAM & ARIA in HTML issues & PRs: N/A
• 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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/acknowledgements.html ( diff )
/custom-elements.html ( diff )

[ja-y-son]

@annevk @rniwa PTAL, thanks!

[annevk]

Oh yeah, this became relevant due to upgrades. Thanks for fixing!