layout: Inform child layout about final block size (#36980)HEADmain

Before this patch it wasn't possibly to simultaneously support intrinsic
min/max sizes and content alignment in the block axis. For example,
block containers only support the former, and flex containers only the
latter.

The reason is that the final block size was decided by the parent
formatting context *after* performing layout, while content alignment is
performed *during* layout.

To address the problem, this introduces the struct `LazySize`, which
contains the data to resolve the final size, except for the intrinsic
size. Thus the parent formatting context can first create a `LazySize`,
then pass it to the child layout so that (if necessary) it can compute
the final size once the intrinsic one is known, and after layout the
parent formatting context uses it to actually size the child.

This PR just provides the functionality that will be used by follow-ups,
but at this point no layout is using the `LazySize` provided by the
parent, so there shouldn't be any behavior change yet.

Testing: Unnecessary (no behavior change)
This is part of #36981 and #36982

Signed-off-by: Oriol Brufau <obrufau@igalia.com>

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