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.
Previously, we always rounded border-widths up when converting them to
device pixels. However, the spec asks us to follow a specific algorithm
to "snap" these values, so that the computed value is snapped.
The difference from before, is that widths of between 0 and 1 device
pixels are rounded up to 1, and and values larger than 1 are rounded
down.
