When calculating the automatic height of a BFC root, we stretch it to
contain the bottommost margin edge of floating boxes.
Before this change, we assumed that floating boxes had coordinates
relative to the BFC root, when they're actually relative to the floating
box's containing block. This may or may not be the BFC root, so we have
to use margin_box_in_ancestor_coordinate_space() to apply offsets from
all boxes in the containing block chain (up to the BFC root).
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