[reading-flow] Can render with carousel

Setting CSS property reading-flow on a carousel scroller is failing
DCHECKs for both the focus navigation and the accessibility tree. To
fix this use case,
1. Relax the OwnedByReadingFlow to be element instead of HTMLElement.
   This is because pseudo-element can be reading flow item scope owners.
2. Change Element::ReadingFlowChildren to add out of flow reading flow
   items in LayoutTreeBuilderTraversal instead of FlatTreeTraversal.
   This will add scroll marker and related pseudo elements.

Change-Id: Icbb523c4f93aff316a8f8bfcbf867e1ec781a1d7
Bug: 398280418, 412401428, 413659838
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6479432
Reviewed-by: Mason Freed <masonf@chromium.org>
Reviewed-by: Aaron Leventhal <aleventhal@chromium.org>
Commit-Queue: Di Zhang <dizhangg@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1451922}