[LayoutNG] Slim down the NGUnpositionedFloat struct.

This removes all the fields which can be passed in via. PositionFloat(s)
There should be no functionality change.

Cq-Include-Trybots: luci.chromium.try:linux_layout_tests_layout_ng
Change-Id: I1b05c0f58bb2aa1ba17374f6600deda56912408c
Bug: 635619
Reviewed-on: https://chromium-review.googlesource.com/1196428
Reviewed-by: Morten Stenshorne <mstensho@chromium.org>
Commit-Queue: Ian Kilpatrick <ikilpatrick@chromium.org>
Cr-Commit-Position: refs/heads/master@{#587539}
7 files changed