LibWeb: Specialize is<DOM::Element>() and is<Layout::Box>()

These two show up in profiles, so let's add specializations for them.

Libraries/LibWeb/Layout/Box.h

@@ -123,6 +123,8 @@ protected:
     Vector<LineBox> m_line_boxes;
 
 private:
+    virtual bool is_box() const final { return true; }
+
     Gfx::FloatPoint m_offset;
     Gfx::FloatSize m_size;
 
@@ -133,3 +135,12 @@ private:
 };
 
 }
+
+namespace AK {
+template<>
+inline bool is<Web::Layout::Box>(const Web::Layout::Node& input)
+{
+    return input.is_box();
+}
+
+}