Inline nodes in our layout tree have a position, so let's show it. By
centralizing the logic for this, block nodes now lose their redundant
'content-size' dump info which is already part of the box model dump.
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.
If a math function resolves to `<length>` or `<percentage>`, then it
will by definition also resolve to `<length-percentage>`. (Same for any
other basic types.) Since we were checking `<length-percentage>` first
and then bailing if no given properties could accept that, math
functions would always fail to match a property that just accepts a non
`-percentage` type.
