[LayoutNG] Construct ConstraintSpace form Physical

Have the NGConstraintSpace constructors take a NGPhysicalConstraintSpace
and make it mandatory. Also add accessors to NGConstraintSpace and allow
a new NGPhysicalConstraintSpace to be constructed from a NGPhysicalSize.

R=ikilpatrick@chromium.org,cbiesinger@chromium.org
BUG=635619
NOTRY=true

Review-Url: https://codereview.chromium.org/2283233002
Cr-Commit-Position: refs/heads/master@{#415161}
4 files changed