[css-grid] Simplify GridResolvedPosition interface

Move static methods from GridResolvedPosition public interface as they
were only used internally.
Also move some static methods from GridSpan that were only used by
Last, remove unused method in GridCoordinate.

Based on a previous patch by Sergio Villar <svillar@igalia.com>

No new tests, no change of behavior.


Review URL: https://codereview.chromium.org/1477203002

Cr-Commit-Position: refs/heads/master@{#361998}
3 files changed