LibWeb: Perform DOMTokenList token validation in the correct order

Author: https://github.com/tcl3
Commit: 0127190dcf
Pull-request: https://github.com/LadybirdBrowser/ladybird/pull/817

Tests/LibWeb/Text/input/DOM/Element-classList.html

@@ -12,6 +12,12 @@
         println(`element.classList after setting to className to "a a b c": "${element.classList.toString()}"`);
         element.className = "   a a b c ";
         println(`element.classList after setting to className to "   a a b c ": "${element.classList.toString()}"`);
+
+        try {
+            element.classList.replace(" ", "");
+        } catch (e) {
+            println(`element.classList.replace(" ", "") throws "${e.name}"`);
+        }
     });
 </script>
 </html>