LibWeb: Make DOM Nodes keep their Document alive

In addition to being reference-counted, all nodes that are part of a
document must also keep the document alive.

This is achieved by adding a second ref-count to the Document object
and incrementing/decrementing it whenever a node is created/destroyed
in that document.

This brings us much closer to a proper DOM lifetime model, although
the JS bindings still need more work.

Libraries/LibWeb/DOM/Document.cpp

@@ -77,6 +77,16 @@ Document::~Document()
 {
 }
 
+void Document::removed_last_ref()
+{
+    ASSERT(!ref_count());
+
+    if (m_referencing_node_count)
+        return;
+
+    delete this;
+}
+
 Origin Document::origin() const
 {
     if (!m_url.is_valid())