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.
This change implements following paragraph from placement algorithm in
the spec:
"If the largest column span among all the items without a definite
column position is larger than the width of the implicit grid, add
columns to the end of the implicit grid to accommodate that column
span."
There were places in the grid implementation code with copies of this
text, but those were completely unrelated to the code where they were
being pasted so I removed them.
