[LayoutNG] Remove small allocation of NGConstraintSpace for floats.

This removes a NGConstraintSpace allocation inside:
NGBlockLayoutAlgorithm::CalculateMarings

This doesn't have a perf impact, but will make it easier to potentially
move margins onto layout results.

Cq-Include-Trybots: luci.chromium.try:linux_layout_tests_layout_ng
Change-Id: I83d6d1f61e2f9bd8f94eed395e759bd01d6cbe78
Bug: 635619
Reviewed-on: https://chromium-review.googlesource.com/1145460
Reviewed-by: Koji Ishii <kojii@chromium.org>
Commit-Queue: Koji Ishii <kojii@chromium.org>
Cr-Commit-Position: refs/heads/master@{#577067}
9 files changed