There's a specific (and thankfully very common!) scenario where we can
actually skip calculating the automatic minimum size for flex items.
In single-line (no wrapping) flex containers, if the sum of all item
flex base sizes is <= the flex container's main size, we know that
none of the items will be shrunk by the layout algorithm.
And so for any flex item with definite main size AND automatic minimum
main size, we can treat the automatic minimum size as 0.
