Instead of storing two JS::Handles into the DOM, we can combine them
into a single one.
If the layout node is anonymous, m_dom_node points to the DOM::Document.
Otherwise, m_dom_node points to the associated DOM node.
The anonymous state is moved to an m_anonymous boolean member.
This cuts the number of JS::Handles created by the layout tree in half
(and shrinks Layout::Node by 8 bytes).
be5a39657e