Simplified layout with fragment items always fails.

As of https://chromium-review.googlesource.com/c/chromium/src/+/3600090
we'd always disallow simplified layout of fragments with items, since
result.succeeded would always be false in this case, since the only way
for FragmentItemsBuilder::AddPreviousItems() to succeed is if the
`end_item` parameter is specified. See
https://source.chromium.org/chromium/chromium/src/+/main:third_party/blink/renderer/core/layout/inline/fragment_items_builder.cc;l=380-384;drc=b55e796ce1b79e1b984741f0ec8f1cc997f9b8f4

No behavior changes intended. This is either performance-neutral, or
slightly positive (since we no longer waste time on copying things just
to throw them away afterwards).

Clean up FragmentItemsBuilder::AddPreviousItems(). `end_item` can no
longer be nullptr

Change-Id: I0609ada6be2a1a3cb8b3467ca7918c91c9a5f07c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6298015
Commit-Queue: Morten Stenshorne <mstensho@chromium.org>
Reviewed-by: Koji Ishii <kojii@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1425076}