LibWeb: Follow the specification steps to serialize a DOMTokenList

This ensures that calling `element.classList.toString()` always
produces the correct value.

Tests/LibWeb/Text/expected/DOM/Element-classList.txt

@@ -1,3 +1,5 @@
  element.classList initial value: ""
 element.classList after setting classList to "a": "a"
 element.classList after setting className to "": ""
+element.classList after setting to className to "a a b c": "a a b c"
+element.classList after setting to className to "   a a b c ": "   a a b c "

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

@@ -8,6 +8,10 @@
         println(`element.classList after setting classList to "a": "${element.classList.toString()}"`);
         element.className = "";
         println(`element.classList after setting className to "": "${element.classList.toString()}"`);
+        element.className = "a a b c";
+        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()}"`);
     });
 </script>
 </html>

Userland/Libraries/LibWeb/DOM/DOMTokenList.cpp

@@ -239,14 +239,20 @@ WebIDL::ExceptionOr<bool> DOMTokenList::supports(StringView token)
     return false;
 }
 
-// https://dom.spec.whatwg.org/#dom-domtokenlist-value
-String DOMTokenList::value() const
+// https://dom.spec.whatwg.org/#concept-ordered-set-serializer
+String DOMTokenList::serialize_ordered_set() const
 {
     StringBuilder builder;
     builder.join(' ', m_token_set);
     return MUST(builder.to_string());
 }
 
+// https://dom.spec.whatwg.org/#dom-domtokenlist-value
+String DOMTokenList::value() const
+{
+    return m_associated_element->get_attribute_value(m_associated_attribute);
+}
+
 // https://dom.spec.whatwg.org/#ref-for-concept-element-attributes-set-value%E2%91%A2
 void DOMTokenList::set_value(String const& value)
 {
@@ -278,7 +284,7 @@ void DOMTokenList::run_update_steps()
         return;
 
     // 2. Set an attribute value for the associated element using associated attribute’s local name and the result of running the ordered set serializer for token set.
-    MUST(associated_element->set_attribute(m_associated_attribute, value()));
+    MUST(associated_element->set_attribute(m_associated_attribute, serialize_ordered_set()));
 }
 
 WebIDL::ExceptionOr<JS::Value> DOMTokenList::item_value(size_t index) const

Userland/Libraries/LibWeb/DOM/DOMTokenList.h

@@ -53,6 +53,8 @@ private:
     WebIDL::ExceptionOr<void> validate_token(StringView token) const;
     void run_update_steps();
 
+    String serialize_ordered_set() const;
+
     JS::NonnullGCPtr<Element> m_associated_element;
     FlyString m_associated_attribute;
     Vector<String> m_token_set;