LibWeb: Add the PageTransitionEvent interface and fire "pageshow" events

We now fire "pageshow" events at the appropriate time during document
loading (done by the parser.)

Note that there are no corresponding "pagehide" events yet.

Userland/Libraries/LibWeb/DOM/Window.cpp

@@ -12,6 +12,7 @@
 #include <LibWeb/DOM/EventDispatcher.h>
 #include <LibWeb/DOM/Timer.h>
 #include <LibWeb/DOM/Window.h>
+#include <LibWeb/HTML/PageTransitionEvent.h>
 #include <LibWeb/HighResolutionTime/Performance.h>
 #include <LibWeb/Layout/InitialContainingBlock.h>
 #include <LibWeb/Page/BrowsingContext.h>
@@ -276,4 +277,22 @@ float Window::scroll_y() const
     return 0;
 }
 
+// https://html.spec.whatwg.org/#fire-a-page-transition-event
+void Window::fire_a_page_transition_event(FlyString event_name, bool persisted)
+{
+    // To fire a page transition event named eventName at a Window window with a boolean persisted,
+    // fire an event named eventName at window, using PageTransitionEvent,
+    // with the persisted attribute initialized to persisted,
+    auto event = HTML::PageTransitionEvent::create(move(event_name), persisted);
+
+    // ...the cancelable attribute intialized to true,
+    event->set_cancelable(true);
+
+    // the bubbles attribute initialized to true,
+    event->set_bubbles(true);
+
+    // and legacy target override flag set.
+    dispatch_event(move(event));
+}
+
 }