[reading-flow] fix out of flow position reading traversal

Out of flow position elements (position: absolute, position: fixed)
are removed from normal document flow and are not included in
ReadingOrderElements. We make sure these elements still get visited
in a reading flow navigation by adding them after the
ReadingOrderelements, in DOM order.
This is similar to how we currently handle display: contents.

AX-Relnotes: No AXObject change needed, we only need to add a test.

Change-Id: Iac3d88d78542f516cf3d1834e126661a75af645b
Bug: 40932006
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5770690
Commit-Queue: Di Zhang <dizhangg@chromium.org>
Reviewed-by: Mason Freed <masonf@chromium.org>
Reviewed-by: Mark Schillaci <mschillaci@google.com>
Auto-Submit: Di Zhang <dizhangg@chromium.org>
Commit-Queue: Mark Schillaci <mschillaci@google.com>
Cr-Commit-Position: refs/heads/main@{#1339217}