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

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

 /*
  * Copyright (C) 2010 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1.  Redistributions of source code must retain the above copyright
  *     notice, this list of conditions and the following disclaimer.
  * 2.  Redistributions in binary form must reproduce the above copyright
  *     notice, this list of conditions and the following disclaimer in the
@@ skipping 35 matching lines @@
   void ClearValueForQuirksMode() { class_names_for_quirks_mode_ = nullptr; }
 
   DECLARE_VIRTUAL_TRACE();
 
  private:
   explicit ClassList(Element*);
 
   bool ContainsInternal(const AtomicString&) const override;
 
   const SpaceSplitString& ClassNames() const;
+  SpaceSplitString& MutableSet() override;
 
   const AtomicString& value() const override {
     return element_->getAttribute(HTMLNames::classAttr);
   }
   void setValue(const AtomicString& value) override {
     element_->setAttribute(HTMLNames::classAttr, value);
+    mutable_set_.Clear();
   }
 
   Member<Element> element_;
   mutable std::unique_ptr<SpaceSplitString> class_names_for_quirks_mode_;
+  SpaceSplitString mutable_set_;
 };
 
 }  // namespace blink
 
 #endif  // ClassList_h