LibWeb: Allow <svg> layout boxes to have children

We can't say that "no replaced boxes can have children", since that
breaks SVG. Instead, let each LayoutNode decide whether it's allowed
to have children.

Fixes #4223.

Libraries/LibWeb/Layout/ReplacedBox.h

@@ -62,6 +62,8 @@ public:
 
     virtual void prepare_for_replaced_layout() { }
 
+    virtual bool can_have_children() const override { return false; }
+
 protected:
     virtual void split_into_lines(Layout::BlockBox& container, LayoutMode) override;