[layoutng] margin: auto support
Add an ApplyAutoMargin function in ng_length_utils and use that from
ng_block_layout_algorithm.
I did not add it to computeMargins because not all callers should take
margin: auto into account. In particular, a child's width: auto computation
needs to treat margin: auto as zero (and, at any rate, does not have a fragment
available yet).
R=glebl@chromium.org,eae@chromium.org
BUG=635619
Review-Url: https://codereview.chromium.org/2399963002
Cr-Commit-Position: refs/heads/master@{#423680}
6 files changed
