layout: Generalize ContainingBlock's block size to a SizeConstraint (#34946)

It used to be an `AuOrAuto`, turning it into a `SizeConstraint` allows
passing the information about the min and max constraints when the
containing block doesn't have a definite block size.

This will be useful for table layout.

Note that in most cases we were already constructing the containing
block from a `SizeConstraint`, but we were calling `to_auto_or()` to
turn it into an `AuOrAuto`.

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

components/layout_2020/flexbox/mod.rs

@@ -191,6 +191,6 @@ impl CachedBlockSizeContribution {
         item_as_containing_block: &ContainingBlock,
     ) -> bool {
         item_as_containing_block.size.inline == self.containing_block_inline_size &&
-            item_as_containing_block.size.block.is_auto()
+            !item_as_containing_block.size.block.is_definite()
     }
 }