Bug 1776860 [wpt PR 34624] - Need post-layout fragments on fragment item cache hit...

Bug 1776860 [wpt PR 34624] - Need post-layout fragments on fragment item cache hit., a=testonly

Automatic update from web-platform-tests
Need post-layout fragments on fragment item cache hit.

We previously used to always get post-layout fragments when cloning a
fragment item, but CL:3677013 changed that, since it's no longer a given
that we always do want the post-layout fragments, and that the callers
rather fetch the post-layout fragments at their own discretion.

Back when I wrote that CL, I didn't realize that
NGFragmentItemsBuilder::AddPreviousItems() relied on getting post-layout
fragments as well (probably because cs.chromium.org failed to list all
call sites to NGFragmentItemsBuilder::ItemWithOffset::ItemWithOffset()).

Bug: 1340168
Change-Id: I5f799aa5305c7588d7a6034b2f8eab33b8e1a89c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3731811
Reviewed-by: Koji Ishii <kojii@chromium.org>
Commit-Queue: Koji Ishii <kojii@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1018694}

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wpt-commits: dfb10692e649e537946ef5db98898649c9010d98
wpt-pr: 34624