[cleanup] Pass the flex container's main-axis available space into FlexLayout as a constraint

https://bugs.webkit.org/show_bug.cgi?id=318611

Reviewed by Antti Koivisto.

FlexLayout computed the flex container's main-axis available space itself: contentBoxLogicalWidth
for row flow, and for column flow the specified logical height via computeLogicalHeight (clearing
and restoring the flex-basis override so it sees the specified height rather than the parent
flex's sizing). That is a container property, and unlike the cross size (which grows as lines
stack) it is fixed before the algorithm runs, so FlexLayout should be handed it rather than reach
back into the container for it.

Move the computation to RenderFlexibleBox and snapshot the result into FlexLayoutConstraints
alongside the flow flags and border/padding; FlexLayout reads m_constraints.mainAxisAvailableSpace
when collecting lines. flexLayoutConstraints() becomes non-const because the column path mutates
the container's override while reading its specified height.

No change in behavior: the value is computed the same way and does not depend on item layout, so
computing it up front (before the flex-base sizing pass) rather than during line collection yields
the same result.

* Source/WebCore/rendering/RenderFlexLayout.cpp:
* Source/WebCore/rendering/RenderFlexLayout.h:
* Source/WebCore/rendering/RenderFlexibleBox.cpp:
* Source/WebCore/rendering/RenderFlexibleBox.h:

Canonical link: https://commits.webkit.org/317278@main