Every LayoutNode indirectly belongs to some Document. For anonymous
LayoutNodes, we simply traverse the parent chain until we find someone
with a Node from which we can get a Document&.
LayoutText can't simply rely on its LayoutNode::rect() for hit testing.
Instead, we have to iterate over the individual runs and see if we're
hitting any of them.
Also, LayoutNode::hit_test() now always recurses into children, since
we can't trust the rect() to tell the truth (inline rects are wrong.)
