Correctly rebuild paginated fragment tree spine.
We failed to find the page fragmentainers inside the paginated root
fragment, since they are "hiding" inside a page box fragment inside a
page container fragment, and thus ended up with a broken spine.
Bug: 349109114
Change-Id: Ic896807cce0715597135948fdef2817b7a60901d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6054790
Commit-Queue: Morten Stenshorne mstensho@chromium.org
Reviewed-by: Koji Ishii kojii@chromium.org
Cr-Commit-Position: refs/heads/main@{#1390262}