[css-grid] Rename GridResolvedPosition to GridPositionsResolver

GridResolvedPosition is not storing a position (track or line) anymore.
Currently it's just a utility class to resolve grid positions from style.
Renamed the class to avoid confusions.

No new tests, no change of behavior.

BUG=444011

Review URL: https://codereview.chromium.org/1648903002

Cr-Commit-Position: refs/heads/master@{#372340}
6 files changed