Bug 1914321 - Build view transition pseudo-element tree. r=view-transitions-reviewers,boris

Reuse the editor's manual NAC machinery for now, and hook it into
StyleChildrenIterator and co.

We might need to slightly tweak the setup for selector-matching, not
sure yet, but that should be fine.

Differential Revision: https://phabricator.services.mozilla.com/D228255

layout/base/PresShell.cpp

@@ -4620,8 +4620,7 @@ MOZ_CAN_RUN_SCRIPT_BOUNDARY void PresShell::ContentRemoved(
   // frame reconstruction.
   nsIContent* oldNextSibling = nullptr;
 
-  // Editor calls into here with NAC via HTMLEditor::DeleteRefToAnonymousNode.
-  // This could be asserted if that caller is fixed.
+  // Editor and view transitions code call into here with NAC.
   if (MOZ_LIKELY(!aChild->IsRootOfNativeAnonymousSubtree())) {
     oldNextSibling = aPreviousSibling ? aPreviousSibling->GetNextSibling()
                                       : container->GetFirstChild();