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

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

 /*
  * 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 42 matching lines @@
   DCHECK(element_->HasClass());
   if (element_->GetDocument().InQuirksMode()) {
     if (!class_names_for_quirks_mode_)
       class_names_for_quirks_mode_ = WTF::WrapUnique(
           new SpaceSplitString(value(), SpaceSplitString::kShouldNotFoldCase));
     return *class_names_for_quirks_mode_.get();
   }
   return element_->ClassNames();
 }
 
+SpaceSplitString& ClassList::MutableSet() {
+  // We can't mutate element_->ClassNames() because it is used to compare class
+  // names before/after class attribute change.
+  if (element_->HasClass())
+    mutable_set_ = ClassNames();
+  else
+    mutable_set_ = SpaceSplitString();
+  return mutable_set_;
+}
+
 DEFINE_TRACE(ClassList) {
   visitor->Trace(element_);
   DOMTokenList::Trace(visitor);
 }
 
 }  // namespace blink