[css-grid] Fix grid-template-columns|rows computed style with content alignment

Computed style of grid-template-columns and grid-template-rows properties
was including the distribution offset because of content alignment.
We should subtract that offset, like we do for the case of gaps,
when we're calculating the computed style.


Review URL: https://codereview.chromium.org/1903183002

Cr-Commit-Position: refs/heads/master@{#388477}
4 files changed