LibWeb: Add a document observer for pageHide/pageShow events

WebDriver will need to "block" until a pageShow event occurs in some
cases.

Userland/Libraries/LibWeb/DOM/Document.cpp

@@ -3045,6 +3045,19 @@ bool Document::anything_is_delaying_the_load_event() const
     return false;
 }
 
+void Document::set_page_showing(bool page_showing)
+{
+    if (m_page_showing == page_showing)
+        return;
+
+    m_page_showing = page_showing;
+
+    for (auto document_observer : m_document_observers) {
+        if (document_observer->document_page_showing_observer())
+            document_observer->document_page_showing_observer()->function()(m_page_showing);
+    }
+}
+
 void Document::invalidate_stacking_context_tree()
 {
     if (auto* paintable_box = this->paintable_box())

Userland/Libraries/LibWeb/DOM/Document.h

@@ -426,7 +426,7 @@ public:
     void decrement_number_of_things_delaying_the_load_event(Badge<DocumentLoadEventDelayer>);
 
     bool page_showing() const { return m_page_showing; }
-    void set_page_showing(bool value) { m_page_showing = value; }
+    void set_page_showing(bool);
 
     bool hidden() const;
     StringView visibility_state() const;

Userland/Libraries/LibWeb/DOM/DocumentObserver.cpp

@@ -27,6 +27,7 @@ void DocumentObserver::visit_edges(Cell::Visitor& visitor)
     visitor.visit(m_document_completely_loaded);
     visitor.visit(m_document_readiness_observer);
     visitor.visit(m_document_visibility_state_observer);
+    visitor.visit(m_document_page_showing_observer);
 }
 
 void DocumentObserver::finalize()
@@ -67,4 +68,12 @@ void DocumentObserver::set_document_visibility_state_observer(Function<void(HTML
         m_document_visibility_state_observer = nullptr;
 }
 
+void DocumentObserver::set_document_page_showing_observer(Function<void(bool)> callback)
+{
+    if (callback)
+        m_document_page_showing_observer = JS::create_heap_function(vm().heap(), move(callback));
+    else
+        m_document_page_showing_observer = nullptr;
+}
+
 }

Userland/Libraries/LibWeb/DOM/DocumentObserver.h

@@ -32,6 +32,9 @@ public:
     [[nodiscard]] JS::GCPtr<JS::HeapFunction<void(HTML::VisibilityState)>> document_visibility_state_observer() const { return m_document_visibility_state_observer; }
     void set_document_visibility_state_observer(Function<void(HTML::VisibilityState)>);
 
+    [[nodiscard]] JS::GCPtr<JS::HeapFunction<void(bool)>> document_page_showing_observer() const { return m_document_page_showing_observer; }
+    void set_document_page_showing_observer(Function<void(bool)>);
+
 private:
     explicit DocumentObserver(JS::Realm&, DOM::Document&);
 
@@ -43,6 +46,7 @@ private:
     JS::GCPtr<JS::HeapFunction<void()>> m_document_completely_loaded;
     JS::GCPtr<JS::HeapFunction<void(HTML::DocumentReadyState)>> m_document_readiness_observer;
     JS::GCPtr<JS::HeapFunction<void(HTML::VisibilityState)>> m_document_visibility_state_observer;
+    JS::GCPtr<JS::HeapFunction<void(bool)>> m_document_page_showing_observer;
 };
 
 }