[LayoutNG] Add NGConstraintSpaceBuilder class.

This patch just introduces the builder without changing the API surface of the
constraint space.

Once everything is moved to using the builder; I'll start changing the API
surface of the constraint space to have PercentageSize() etc.

BUG=635619

Review-Url: https://codereview.chromium.org/2446243003
Cr-Commit-Position: refs/heads/master@{#427769}
5 files changed