[css-lists] Outside marker when children are blocks without line boxes

[kojiishi]

This is for info-sharing as we worked on re-implementing lists in Blink.

When children of a list item are blocks without line boxes, it looks like each browser lays out differently.

Tests@codepen

1. The first one is easy, just an empty list item. 4 browsers create an empty line box.
2. If the block child has a line box, still easy. 4 browsers look interoperable.
3. When the block child is empty, Gecko computes the height to zero, while other 3 assumes there's an empty line box in the block.
4. When the block is empty but has height, and another block with a line box follow, Edge inserts an empty line box before the block child, while other 3 positions the list marker at the first line box.
5. When the block is empty but has height, and no other children, Gecko positions the list marker at the top of the list item, while other 3 creates an empty line box.

Our current new implementation is somewhat in between the 4 engines.

[kojiishi]

/cc @FremyCompany @upsuper @cathiechen

[FremyCompany]

Ideally, I think we would want to preserve Edge behavior, to the exception of 4 where we would instead position the marker out of flow (but we would not try to match the line box position, we would just top align a line box containing the bullet).