To make this possible, I also had to give each LayoutNode a Document&
so it can resolve document-specific colors correctly. There's probably
ways to avoid having this extra member by resolving colors later, but
this works for now.
This patch implements a simple <object> element with fallback content.
If the URL from the data attribute fails to load (including 404),
we render the DOM tree inside the <object> as fallback content.
This works by generating a different layout tree for the <object>
depending on the state and success of the data load. Since we cannot
currently do incremental layout tree updates, we have to force a
complete layout tree rebuild when the resource load finishes/fails.
