[layoutng] Make RunOldLayout respect IsFixedSize

If IsFixedSize is true for a dimension, sets the override size
accordingly, matching the code in ng_length_utils.

Also adds a comment to ng_constraint_space.h for how this works
(could've sworn that we already had such a comment)

R=eae@chromium.org,atotic@chromium.org
BUG=635619

Review-Url: https://codereview.chromium.org/2611033002
Cr-Commit-Position: refs/heads/master@{#441713}
2 files changed