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.
An earlier variant of the commit following this one introduced a
regression for the behavior tested here, but did not fail any in-tree
tests. So lets add an explicit regression test to make that easier to
catch in the future.
