[css-writing-modes] Available space in ortho flows needs to care about "definite" container size, not "fixed"

[tabatkins]

In https://drafts.csswg.org/css-writing-modes/#orthogonal-auto, the algorithm checks if a container has a "fixed" size in a few places.

All of these cases should instead be checking for a "definite" size; in cases like a flex item whose size was "auto" but is treated as definite per the Flex Layout rules, ortho flows in that item should respect that size (rather than falling thru to the viewport-based size).