[CSS Grid Layout] Implement auto-margins alignment of grid items

The Grid Layout spec states that we must center items with auto margins,
so it will absorb any positive free space, both horiontal and vertical.

https://drafts.csswg.org/css-grid/#auto-margins

Besides, auto-margins imply that any alignment property in the same axis
will have no effect

BUG=79180, 249451, 376823

Review URL: https://codereview.chromium.org/1298623002

Cr-Commit-Position: refs/heads/master@{#351796}