[layoutng] Add a function to compute the min- and max-content contribution

This will be used inside of ComputeMinAndMaxContentSizes for computing the
child's contribution to the parent's min and max content size.

R=ikilpatrick@chromium.org
BUG=635619

Review-Url: https://codereview.chromium.org/2583063004
Cr-Commit-Position: refs/heads/master@{#440105}
5 files changed