LibWeb: Make HTMLImageElement loads delay the document load event

This is something we're supposed to do according to the HTML spec.
Note that image loading is currently completely ad-hoc, and this just
adds a simple DocumentLoadEventDelayer to the existing implementation.

This will allow us to use images in layout tests, which rely on the
document load event firing at a predictable time.

Userland/Libraries/LibWeb/HTML/HTMLImageElement.h

@@ -9,6 +9,7 @@
 #include <AK/ByteBuffer.h>
 #include <AK/OwnPtr.h>
 #include <LibGfx/Forward.h>
+#include <LibWeb/DOM/DocumentLoadEventDelayer.h>
 #include <LibWeb/HTML/FormAssociatedElement.h>
 #include <LibWeb/HTML/HTMLElement.h>
 #include <LibWeb/Loader/ImageLoader.h>
@@ -54,6 +55,8 @@ private:
     virtual JS::GCPtr<Layout::Node> create_layout_node(NonnullRefPtr<CSS::StyleProperties>) override;
 
     ImageLoader m_image_loader;
+
+    Optional<DOM::DocumentLoadEventDelayer> m_load_event_delayer;
 };
 
 }