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

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

Index: third_party/WebKit/Source/core/html/ClassList.h
diff --git a/third_party/WebKit/Source/core/html/ClassList.h b/third_party/WebKit/Source/core/html/ClassList.h
index 79d3f20a8e77e6346b559e5345d9b2875b832dfc..3dcc1df572a20e071f27b56b5a9fac882f78f991 100644
--- a/third_party/WebKit/Source/core/html/ClassList.h
+++ b/third_party/WebKit/Source/core/html/ClassList.h
@@ -53,16 +53,19 @@ class ClassList final : public DOMTokenList {
   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