LibWeb: Let subframes propagate paint invalidations via host element

When a paint invalidation occurs inside a subframe, it bubbles up to
Frame::set_needs_display(). From there, we call PageView if this is
the main frame, or otherwise invalidate the subframe host element.

Libraries/LibWeb/DOM/HTMLIFrameElement.cpp

@@ -79,11 +79,6 @@ void HTMLIFrameElement::load_src(const String& value)
         return;
     }
 
-    m_hosted_frame->on_set_needs_display = [this](auto&) {
-        if (layout_node())
-            layout_node()->set_needs_display();
-    };
-
     m_hosted_frame->loader().load(url);
 }