[LayoutNG] Fix inserting a child into an anonymous first-line inline.

This fixes a bug that was brought up in:
https://chromium-review.googlesource.com/c/chromium/src/+/1405884

Effectively we now have an anonymous inline wrapper representing the
first-line style. We also have checks in the current codebase relating
to a similar concept with display:contents. Makes sure this branch
is only hit for the display:contents case.

Bug: 636993
Cq-Include-Trybots: luci.chromium.try:linux_layout_tests_layout_ng
Change-Id: I0271952aa981d861cc7a33e646b5594efe22da1f
Reviewed-on: https://chromium-review.googlesource.com/c/1413634
Commit-Queue: Ian Kilpatrick <ikilpatrick@chromium.org>
Reviewed-by: Koji Ishii <kojii@chromium.org>
Reviewed-by: Morten Stenshorne <mstensho@chromium.org>
Reviewed-by: Rune Lillesveen <futhark@chromium.org>
Cr-Commit-Position: refs/heads/master@{#623239}
3 files changed