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())

Libraries/LibWeb/DOM/Document.h

@@ -177,9 +177,27 @@ public:
     const String& ready_state() const { return m_ready_state; }
     void set_ready_state(const String&);
 
+    void ref_from_node(Badge<Node>)
+    {
+        ++m_referencing_node_count;
+    }
+
+    void unref_from_node(Badge<Node>)
+    {
+        ASSERT(m_referencing_node_count);
+        --m_referencing_node_count;
+        if (!m_referencing_node_count && !ref_count()) {
+            removed_last_ref();
+        }
+    }
+
+    void removed_last_ref();
+
 private:
     virtual RefPtr<LayoutNode> create_layout_node(const CSS::StyleProperties* parent_style) override;
 
+    unsigned m_referencing_node_count { 0 };
+
     OwnPtr<CSS::StyleResolver> m_style_resolver;
     RefPtr<CSS::StyleSheetList> m_style_sheets;
     RefPtr<Node> m_hovered_node;

Libraries/LibWeb/DOM/Node.cpp

@@ -206,4 +206,13 @@ Bindings::EventTargetWrapper* Node::create_wrapper(JS::GlobalObject& global_obje
     return wrap(global_object, *this);
 }
 
+void Node::removed_last_ref()
+{
+    if (is<Document>(*this)) {
+        downcast<Document>(*this).removed_last_ref();
+        return;
+    }
+    delete this;
+}
+
 }

Libraries/LibWeb/DOM/Node.h

@@ -65,6 +65,8 @@ public:
 
     virtual ~Node();
 
+    void removed_last_ref();
+
     NodeType type() const { return m_type; }
     bool is_element() const { return type() == NodeType::ELEMENT_NODE; }
     bool is_text() const { return type() == NodeType::TEXT_NODE; }