In order to support non-xul-flex elements from various layout code, we want to reduce reliance on nsFrame::IsXULBoxFrame. For instance, it's called by other xul layout types (stacks, decks, menus, etc) that wouldn't find nearby elements.

One option is to expand these checks to include emulated XUL flex children (bug 1493962), but really we should also support non-emulated or CSS flex in order to allow us to experiment with using other (faster) layout for UI when it's appropriate.

One first step might be to eliminate some of these calls. For example
the call at https://searchfox.org/mozilla-central/rev/c606cdd6d014fee4034da1702d484c0d41b604c9/layout/xul/nsDeckFrame.cpp#221
doesn't actually require the child to be a xul box and can be any type
of frame.

See callers at https://searchfox.org/mozilla-central/search?q=GetChildXULBox&path=. Some of these are in xul flex or grid layout implementations and can be ignored, but others should be able to support all kinds of children.

We have very few of these left.