LibWeb: Add JSON serialization for nested browsing contexts

This changes allows for nested browser contexts to be embedded in the
serialized JSON of their container element (like `iframe`) and enables
their inspection in the DOM Inspector.

Userland/Libraries/LibWeb/DOM/Node.cpp

@@ -24,6 +24,7 @@
 #include <LibWeb/DOM/Node.h>
 #include <LibWeb/DOM/ProcessingInstruction.h>
 #include <LibWeb/DOM/ShadowRoot.h>
+#include <LibWeb/HTML/BrowsingContextContainer.h>
 #include <LibWeb/HTML/HTMLAnchorElement.h>
 #include <LibWeb/HTML/Parser/HTMLParser.h>
 #include <LibWeb/Layout/InitialContainingBlock.h>
@@ -702,13 +703,22 @@ void Node::serialize_tree_as_json(JsonObjectSerializer<StringBuilder>& object) c
     } else if (is_element()) {
         object.add("type", "element");
 
-        auto element = static_cast<DOM::Element const*>(this);
+        auto const* element = static_cast<DOM::Element const*>(this);
         if (element->has_attributes()) {
             auto attributes = object.add_object("attributes");
             element->for_each_attribute([&attributes](auto& name, auto& value) {
                 attributes.add(name, value);
             });
         }
+
+        if (element->is_browsing_context_container()) {
+            auto const* container = static_cast<HTML::BrowsingContextContainer const*>(element);
+            if (auto const* content_document = container->content_document()) {
+                auto children = object.add_array("children");
+                JsonObjectSerializer<StringBuilder> content_document_object = children.add_object();
+                content_document->serialize_tree_as_json(content_document_object);
+            }
+        }
     } else if (is_text()) {
         object.add("type", "text");
 

Userland/Libraries/LibWeb/DOM/Node.h

@@ -84,6 +84,7 @@ public:
 
     virtual bool is_html_html_element() const { return false; }
     virtual bool is_html_template_element() const { return false; }
+    virtual bool is_browsing_context_container() const { return false; }
 
     ExceptionOr<NonnullRefPtr<Node>> pre_insert(NonnullRefPtr<Node>, RefPtr<Node>);
     ExceptionOr<NonnullRefPtr<Node>> pre_remove(NonnullRefPtr<Node>);

Userland/Libraries/LibWeb/Forward.h

@@ -116,6 +116,7 @@ class DOMRectReadOnly;
 
 namespace Web::HTML {
 class BrowsingContext;
+class BrowsingContextContainer;
 class CanvasRenderingContext2D;
 class CloseEvent;
 class DOMParser;

Userland/Libraries/LibWeb/HTML/BrowsingContextContainer.h

@@ -29,6 +29,14 @@ public:
 
 protected:
     RefPtr<BrowsingContext> m_nested_browsing_context;
+
+private:
+    virtual bool is_browsing_context_container() const override { return true; }
 };
 
 }
+
+namespace Web::DOM {
+template<>
+inline bool Node::fast_is<HTML::BrowsingContextContainer>() const { return is_browsing_context_container(); }
+}