LibWeb: Don't generate layout nodes for DOM inside replaced elements

Before this change, we'd show both a <canvas>, and any fallback content
inside the <canvas> for browsers that don't support <canvas>. :^)

Libraries/LibWeb/Layout/TreeBuilder.cpp

@@ -129,6 +129,10 @@ void TreeBuilder::create_layout_tree(DOM::Node& dom_node)
         }
     }
 
+    // Ignore fallback content inside replaced elements.
+    if (layout_node->is_replaced())
+        return;
+
     if (dom_node.has_children()) {
         push_parent(*layout_node);
         downcast<DOM::ParentNode>(dom_node).for_each_child([&](auto& dom_child) {