Introduce a PackedCheckedPtr and use it for RenderObject::m_next, to gain extra safety without increasing the size of renderers.
Pull request: https://github.com/WebKit/WebKit/pull/19387
Committed 269629@main (d4e1b3b3e736): <https://commits.webkit.org/269629@main> Reviewed commits have been landed. Closing PR #19387 and removing active labels.
<rdar://problem/117309003>