The bulk of Document::updateIntersectionObservations() should be moved into IntersectionObserver.cpp
<rdar://problem/112176104>
Pull request: https://github.com/WebKit/WebKit/pull/15807
Committed 266034@main (3b73b01fe389): <https://commits.webkit.org/266034@main> Reviewed commits have been landed. Closing PR #15807 and removing active labels.