[LayoutNG] Keep child NGLayoutResult in NGLineBoxFragmentBuilder

This patch changes NGLineBoxFragmentBuilder to keep child
NGLayoutResult until it finalizes children offsets and descendant
box tree structure.

This is needed to give the final offsets (relative to the line box)
to e.g., out of flow descendants.

Bug: 635619
Cq-Include-Trybots: master.tryserver.chromium.linux:linux_layout_tests_layout_ng
Change-Id: I850fec48bc32262437965bec7cd3cbf1df2e660d
Reviewed-on: https://chromium-review.googlesource.com/720590
Commit-Queue: Koji Ishii <kojii@chromium.org>
Reviewed-by: Ian Kilpatrick <ikilpatrick@chromium.org>
Cr-Commit-Position: refs/heads/master@{#510612}
10 files changed