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This implements `document.scrollingElement`
(https://drafts.csswg.org/cssom-view/#dom-document-scrollingelement).
---
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- [x] `./mach build -d` does not report any errors
- [x] `./mach test-tidy` does not report any errors
- [x] These changes fix#35700
- [x] There are tests for these changes
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Signed-off-by: JimmyDdotEXE <50691404+JimmyDdotEXE@users.noreply.github.com>
