When restructuring inline nodes because of a block element insertion
during the layout tree build, we might end up with a `display: contents`
element in the ancestor stack that is not part of the actual layout
tree, since it's never actually used as a parent for any node. Because
we were only rewinding the ancestor stack with actual new layout nodes,
it became corrupted and layout nodes were added to the wrong parent.
This new logic leaves the ancestor stack intact, only replacing layout
nodes whenever a new one is created.
Fixes the sidebar on https://reddit.com.
Fixes#3590.
Our layout tree requires that all containers either have inline or
non-inline children. In order to support the layout of non-inline
elements inside inline elements, we need to do a bit of tree
restructuring. It effectively simulates temporarily closing all inline
nodes, appending the block element, and resumes appending to the last
open inline node.
The acid1.txt expectation needed to be updated to reflect the fact that
we now hoist its <p> elements out of the inline <form> they were in.
Visually, the before and after situations for acid1.html are identical.
