Compute margin block start for 1st block in LayoutNG root constraint space

It will correctly compute margin block start for the 1st block in
LayoutNG's root constraint space. This is because margins of the root
element's box do not collapse https://www.w3.org/TR/CSS2/box.html#collapsing-margins

BUG=635619

Review-Url: https://codereview.chromium.org/2368153003
Cr-Commit-Position: refs/heads/master@{#421907}
5 files changed