[LFC Display] Store the display tree as a tree of stacking items
Created attachment 414959 [details] Patch
Created attachment 415002 [details] Patch
Created attachment 415003 [details] Patch
Created attachment 415004 [details] Patch
ChangeLog entry in Source/WebCore/ChangeLog contains OOPS!.
<rdar://problem/71984765>
https://trac.webkit.org/r270474