[LayoutNG] Move legacy tree population logic

Move the logic in NGInlineNode that populates the legacy layout tree out
of NGInlineNode and into the new NGInlineNodeLegacy class.

Bug: 591099
Cq-Include-Trybots: master.tryserver.chromium.linux:linux_layout_tests_layout_ng
Change-Id: I7d228a1c9b1c28f301838336ecc1348eacb6e663
Reviewed-on: https://chromium-review.googlesource.com/898003
Reviewed-by: Koji Ishii <kojii@chromium.org>
Commit-Queue: Emil A Eklund <eae@chromium.org>
Cr-Commit-Position: refs/heads/master@{#533876}
7 files changed