Sync web-platform-tests PR 34773 into mozilla-central (this bug is closed when the sync is complete).

PR: https://github.com/web-platform-tests/wpt/pull/34773
Details from upstream follow.

Xianzhu Wang <wangxianzhu@chromium.org> wrote:

Follow containing block chain when finding scroll container for sticky

When a sticky position element is contained by a fixed or absolute
position element, the scroll container of the sticky position element
should follow the containing block chain of the fixed or absolute
position element. Previously we followed the DOM hierarchy, causing
weird behavior in the case, e.g. the sticky element moves in the
opposite direction when the non-containing ancestor scroller scrolls.

Bug: 881555
Change-Id: I11994dfea2910283f06662558a8445362f15bca7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3752748
Reviewed-by: Robert Flack \<flackr@chromium.org>
Commit-Queue: Xianzhu Wang \<wangxianzhu@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1025467}

https://hg.mozilla.org/mozilla-central/rev/3e47d85da58f