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.
This patch just adds the new root paintable and updates the tests
expectations. The next patch will move painting logic from the layout
viewport to the paint viewport.
