[LayoutNG] Switch LineSegment to using LayoutUnit's instead of floats.

There should be no behaviour change. This is done so we can modify the
LineSegment in LayoutNG to be in the correct coordinate system.

Bug: 635619
Change-Id: I4be2b099fc0731e0927b8a548a960ff00d9ddf9c
Reviewed-on: https://chromium-review.googlesource.com/1050487
Commit-Queue: Ian Kilpatrick <ikilpatrick@chromium.org>
Reviewed-by: Morten Stenshorne <mstensho@chromium.org>
Reviewed-by: Koji Ishii <kojii@chromium.org>
Cr-Commit-Position: refs/heads/master@{#557607}
7 files changed