While calculating the minimum size for main min/max size violations
we were flooring the min size to 0 if the item doesn't have a min
main size. Instead of that determine the intrinsic min main size
of that element.
This fixes the flex: 80% 0 4/1/0 test case in the flex.html
test page.
This case was missed in a previous commit that added the
determine_min_main_size_of_child function
7be57c322e