Instead of dumping only the single paintable passed into the function,
iterate over all paintables attached to its layout node. This makes
detached/disconnected paintables visible in the dump output, which is
useful for debugging incremental layout issues.
Rebaseline content-image-simple layout test: the anonymous InlineNode
for ::before content spanning multiple lines legitimately has a
separate PaintableWithLines per line, which now all appear in the dump.
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 expands our generated content support beyond single strings
to lists of strings and/or images.
Pseudo-elements like ::before and ::after can now use content:url(...)
to insert anonymous image boxes into the layout tree.
This is heavily used in Google Docs for UI elements.
