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

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

Index: third_party/WebKit/Source/core/html/ClassList.cpp
diff --git a/third_party/WebKit/Source/core/html/ClassList.cpp b/third_party/WebKit/Source/core/html/ClassList.cpp
index 5a749472c108ebb7740192554b389aafb9c618bf..d38aed24471a068ccf5e18d65926c93ecf0b072a 100644
--- a/third_party/WebKit/Source/core/html/ClassList.cpp
+++ b/third_party/WebKit/Source/core/html/ClassList.cpp
@@ -60,6 +60,16 @@ const SpaceSplitString& ClassList::ClassNames() const {
   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);