Add Flexbox last baseline alignment for block flow items

[Sammy Gill]

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.

[Radar WebKit Bug Importer]

<rdar://problem/99506835>

[Sammy Gill]

Pull request: https://github.com/WebKit/WebKit/pull/4067

[EWS]

Committed 255383@main (a7e014a01430): <https://commits.webkit.org/255383@main>

Reviewed commits have been landed. Closing PR #4067 and removing active labels.

[Tim Nguyen (:ntim)]

*** Bug 243523 has been marked as a duplicate of this bug. ***