Bug 244734

From CSS Box Alignment Module Level 3:

The first/last baseline set of a block container is generated from the dominant first/last baseline of the first/last in-flow line box in the block container, or is taken from the first/last in-flow block-level child in the block container that contributes a set of first/last baselines, whichever comes first/last. If there is no such line box or child, then the block container has no baseline set.

However, for legacy reasons a block-level or inline-level scroll container always has a last baseline set, whose baselines all correspond to its block-end margin edge.

Need to implement the ability to get the last line's baseline value for block flow elements so that they may participate in last baseline alignment.