LibWeb: Use DocumentLoadEventDelayer to delay load event in Navigable

Use a delayer object that actually delays load event for child
navigables instead of boolean flag that does nothing.

Userland/Libraries/LibWeb/HTML/Navigable.cpp

@@ -90,6 +90,17 @@ void Navigable::visit_edges(Cell::Visitor& visitor)
     visitor.visit(m_container);
 }
 
+void Navigable::set_delaying_load_events(bool value)
+{
+    if (value) {
+        auto document = container_document();
+        VERIFY(document);
+        m_delaying_the_load_event.emplace(*document);
+    } else {
+        m_delaying_the_load_event.clear();
+    }
+}
+
 JS::GCPtr<Navigable> Navigable::navigable_with_active_document(JS::NonnullGCPtr<DOM::Document> document)
 {
     for (auto* navigable : all_navigables()) {

Userland/Libraries/LibWeb/HTML/Navigable.h

@@ -10,6 +10,7 @@
 #include <AK/String.h>
 #include <LibJS/Heap/Cell.h>
 #include <LibWeb/Bindings/NavigationPrototype.h>
+#include <LibWeb/DOM/DocumentLoadEventDelayer.h>
 #include <LibWeb/Forward.h>
 #include <LibWeb/HTML/ActivateTab.h>
 #include <LibWeb/HTML/HistoryHandlingBehavior.h>
@@ -56,8 +57,7 @@ public:
     bool is_closing() const { return m_closing; }
     void set_closing(bool value) { m_closing = value; }
 
-    bool is_delaying_load_events() const { return m_delaying_load_events; }
-    void set_delaying_load_events(bool value) { m_delaying_load_events = value; }
+    void set_delaying_load_events(bool value);
 
     JS::GCPtr<SessionHistoryEntry> active_session_history_entry() const { return m_active_session_history_entry; }
     void set_active_session_history_entry(JS::GCPtr<SessionHistoryEntry> entry) { m_active_session_history_entry = entry; }
@@ -160,7 +160,7 @@ private:
     bool m_closing { false };
 
     // https://html.spec.whatwg.org/multipage/document-sequences.html#delaying-load-events-mode
-    bool m_delaying_load_events { false };
+    Optional<DOM::DocumentLoadEventDelayer> m_delaying_the_load_event;
 
     // Implied link between navigable and its container.
     JS::GCPtr<NavigableContainer> m_container;