[@container] Container lookup in flat tree
The spec changed from looking up shadow-including ancestors to flat tree
ancestors for finding the container being queried by a container query.
Bug: 340876720
Change-Id: Id069f09c311fd49256c73bcf6d4b9b420ebe6c7c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5625932
Commit-Queue: Rune Lillesveen futhark@chromium.org
Reviewed-by: Anders Hartvoll Ruud andruud@chromium.org
Cr-Commit-Position: refs/heads/main@{#1315123}