Rewrite ForAllBoxFragmentDescendants callback code.

Just use a regular listener class instead of base::FunctionRef. Split
fragment entry and culled inline handling into two separate callbacks.
Make handling of culled inlines optional, since this will probably only
be needed by ColumnPseudoElement.

This is preparation for yet another callback, that is to be called when
exiting a fragment (i.e. after having walked its subtree).

Also remove TreeOrderedList usage from ColumnPseudoElement. We only need
to know the very first element, so no need to keep a list.

No behavior changes intended.

Change-Id: I84cb048cd97c294881e5f56f0241e997dc2691f7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6494571
Commit-Queue: Morten Stenshorne <mstensho@chromium.org>
Reviewed-by: Ian Kilpatrick <ikilpatrick@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1456098}