[LayoutNG] Introduce NGConstraintSpace::RareData.

This moves fields relating to:
 - Floats clearance
 - Fragmentation
 - Margin struts
 - Weird percentages

... to a rare data member. This decreases partition alloc usage on
system_health.memory_desktop by approximately 6-7%, at the cost of
0-6% on most benchmarks.

Change-Id: Ifaeaf5d874efcd13ce9eff785d8093d24637de86
Bug: 635619
Reviewed-on: https://chromium-review.googlesource.com/c/1323803
Commit-Queue: Ian Kilpatrick <ikilpatrick@chromium.org>
Reviewed-by: Morten Stenshorne <mstensho@chromium.org>
Reviewed-by: Christian Biesinger <cbiesinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#609030}
14 files changed