When modifying the button layout during tree building to use flex for
vertical alignment, let's explicitly set the button box's children to
be non-inline. It doesn't make sense to layout the button as an IFC
when its only child is a flex container.
Using flex layout inside button solves the issue with wrongly calculated
height when it has: pseudo element and whitespaces inside.
Also using flex instead of a table layout allows for the same vertical
alignment but with fewer layout nodes: a flex container and anonymous
wrapper for content instead of a table wrapper, table, row, and cell.
