[LayoutNG] Add Vector<NGPositionedFloat> to NGLayoutResult

This also changes NGLineBoxFragmentBuilder to return a NGLayoutResult,
instead of just a NGLineBoxPhysicalFragment.

The layout result isn't used yet, but will be soon \o/

Bug: 635619
Cq-Include-Trybots: master.tryserver.chromium.linux:linux_layout_tests_layout_ng
Change-Id: Id8bde11d7b2a7f3364d575e076b485bf2219e669
Reviewed-on: https://chromium-review.googlesource.com/721685
Commit-Queue: Ian Kilpatrick <ikilpatrick@chromium.org>
Reviewed-by: Koji Ishii <kojii@chromium.org>
Cr-Commit-Position: refs/heads/master@{#509442}
5 files changed