[css-grid] Simplify method to resolve auto-placed items

Refactor the method to resolve auto-placed items,
for which we're only interested in knowing the span size.

Adapt the calls to use the span size instead of a GridSpan.

No new tests, no change of behavior.


Review URL: https://codereview.chromium.org/1496863004

Cr-Commit-Position: refs/heads/master@{#363457}
3 files changed