[LayoutNG] Change PositionFloats signature

This patch changes PositionFloats to have
NGPositionedFloatVector* to return the positioned floats,
instead of it to return the vector.

NGPositionedFloatVector has inline buffer that returning
it involves a bit of memory copy overhead.

Pinpoint shows slight improvement. We could see them within
errors, but mostly positive, and running twice shows similar
trends.
https://pinpoint-dot-chromeperf.appspot.com/job/11f3134ce40000

Bug: 635619
Cq-Include-Trybots: luci.chromium.try:linux_layout_tests_layout_ng
Change-Id: I29c23e370b398f54a9acc1745f37ea9ffcb1b2fc
Reviewed-on: https://chromium-review.googlesource.com/c/1282689
Reviewed-by: Ian Kilpatrick <ikilpatrick@chromium.org>
Reviewed-by: Christian Biesinger <cbiesinger@chromium.org>
Commit-Queue: Koji Ishii <kojii@chromium.org>
Cr-Commit-Position: refs/heads/master@{#600101}
4 files changed