[LayoutNG] Set inline capacity (to 1) for unpositioned float vectors.

Also remove RefCounted for NGUnpositionedFloat objects.
This shows a 2-5% perf improvement of float performance tests.

Cq-Include-Trybots: luci.chromium.try:linux_layout_tests_layout_ng
Change-Id: I6bf42c7bb076dbdafbc74cf8f7667c5ea5eb6357
Bug: 635619
Reviewed-on: https://chromium-review.googlesource.com/1193881
Commit-Queue: Ian Kilpatrick <ikilpatrick@chromium.org>
Reviewed-by: Christian Biesinger <cbiesinger@chromium.org>
Reviewed-by: Morten Stenshorne <mstensho@chromium.org>
Cr-Commit-Position: refs/heads/master@{#587517}
13 files changed