[LayoutNG] Remove logic from NGConstraintSpace constructor.

Instead make the NGConstraintSpaceBuilder to directly write into the
NGConstraintSpace members. Along with a (very) slight perf win (easier
on the stack), it makes it easier for the rare_data_ patch to follow
this one.

There should be no behaviour change.

Bug: 635619
Change-Id: I0f75166df760ed03fb8f0ab3099c5b12760547ed
Reviewed-on: https://chromium-review.googlesource.com/c/1321571
Commit-Queue: Ian Kilpatrick <ikilpatrick@chromium.org>
Reviewed-by: Morten Stenshorne <mstensho@chromium.org>
Cr-Commit-Position: refs/heads/master@{#607668}
23 files changed