this is in preparation for support different types of inline level boxes.
Created attachment 421844 [details] Patch
Created attachment 421848 [details] Patch
Created attachment 421900 [details] Patch
Committed r273723: <https://commits.webkit.org/r273723> All reviewed patches have been landed. Closing bug and clearing flags on attachment 421900 [details].
<rdar://problem/74908784>