When the grid layout gets to
`resolve_items_box_metrics(GridDimension::Column)`, we've already
determined the width of each column. However, the widths of the
individual grid items themselves haven't been set. Rather than using
`get_available_space_for_item()`, which returns an indefinite size if
an item's width/height hasn't been set, we should use the already
known track width as the available size to calculate the fit-content
width.
