Part 2 - Consider IB-split when computing position-visibility and walking the ASR chain.

The DisplayPortUtils.cpp modification is needed to prevent this
assertion [1]:

```
Assertion failure:
nsLayoutUtils::IsProperAncestorFrameConsideringContinuations(
  aLimitAncestor, asrFrame.mFrame)
```

Teach
`nsLayoutUtils::IsAncestorFrameCrossDocInProcessConsideringContinuations()`
the concept of IB-split so that `DOMIntersectionObserver` [2] works
when calling from `ComputePositionVisibility()`.

[1] https://searchfox.org/firefox-main/rev/935d871d99c5ccb94a8a8b087241d08b4fe1deae/layout/base/DisplayPortUtils.cpp#1272-1273
[2] https://searchfox.org/firefox-main/rev/935d871d99c5ccb94a8a8b087241d08b4fe1deae/dom/base/DOMIntersectionObserver.cpp#817-825

Differential Revision: https://phabricator.services.mozilla.com/D322477

bugzilla-url: https://bugzilla.mozilla.org/show_bug.cgi?id=2066154
gecko-commit: 71e45e828212344892954b9937ce75d9c93a05ed
gecko-commit-git: d9f248df0752d738e1e3cb015190fa0af9296020
gecko-reviewers: layout-reviewers, emilio