DOMTokenList: Update serialization algorithm on add()/remove()

third_party/WebKit/Source/core/html/RelList.h

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef RelList_h
 #define RelList_h
 
 #include "core/HTMLNames.h"
 #include "core/dom/DOMTokenList.h"
 #include "core/dom/Element.h"
@@ skipping 10 matching lines @@
 
   void SetRelValues(const AtomicString&);
 
   DECLARE_VIRTUAL_TRACE();
 
  private:
   explicit RelList(Element*);
 
   bool ContainsInternal(const AtomicString&) const override;
 
+  SpaceSplitString& MutableSet() override { return rel_values_; }
   const AtomicString& value() const override {
     return element_->getAttribute(HTMLNames::relAttr);
   }
   void setValue(const AtomicString& value) override {
     element_->setAttribute(HTMLNames::relAttr, value);
   }
 
   bool ValidateTokenValue(const AtomicString&, ExceptionState&) const override;
 
   Member<Element> element_;
   SpaceSplitString rel_values_;
 };
 
 }  // namespace blink
 
 #endif  // RelList_h