[whatwg/dom] Include cloning keep custom element registry null while cloning shadow root (PR #1445)

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Make sure cloning a shadow root also clone the shadow root variable `keep custom element registry null`. This behavior is already being tested in `wpt/custom-elements/registries/element-mutation.html`. The test will be moved in https://github.com/web-platform-tests/wpt/pull/56423/ but the coverage should remain to be the same.

- [v] At least two implementers are interested (and none opposed):
   * Chromium
   * WebKit
- [v] [Tests](https://github.com/web-platform-tests/wpt) are written and can be reviewed and commented upon at:
   * https://github.com/web-platform-tests/wpt/pull/56423/
- [v] [Implementation bugs](https://github.com/whatwg/meta/blob/main/MAINTAINERS.md#handling-pull-requests) 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

- [v] [MDN issue](https://github.com/whatwg/meta/blob/main/MAINTAINERS.md#handling-pull-requests) is filed: N/A
- [v] The top of this comment includes a [clear commit message](https://github.com/whatwg/meta/blob/main/COMMITTING.md) to use. <!-- If you created this PR from a single commit, Github copied its message. Otherwise, you need to add a commit message yourself. -->

(See [WHATWG Working Mode: Changes](https://whatwg.org/working-mode#changes) for more details.)

You can view, comment on, or merge this pull request online at:

  https://github.com/whatwg/dom/pull/1445

-- Commit Summary --

  * Include cloning keep custom element registry null while cloning

-- File Changes --

    M dom.bs (4)

-- Patch Links --

https://github.com/whatwg/dom/pull/1445.patch
https://github.com/whatwg/dom/pull/1445.diff

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