[layoutng] Move and rename NGBoxMargins to NGBoxStrut

I want to reuse this struct for borders and padding as well, so I want
to rename it to a more generic name, and it belongs more to ng_units.h

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

Review-Url: https://codereview.chromium.org/2281763002
Cr-Commit-Position: refs/heads/master@{#414658}
6 files changed