Dispatch change event for input type=checkbox when checked but value is null

Source/core/html/forms/CheckboxInputType.cpp

 /*
  * Copyright (C) 2010 Google Inc. All rights reserved.
  * Copyright (C) 2011 Apple 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:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
@@ skipping 89 matching lines @@
 bool CheckboxInputType::isCheckbox() const
 {
     return true;
 }
 
 bool CheckboxInputType::supportsIndeterminateAppearance() const
 {
     return true;
 }
 
+bool CheckboxInputType::shouldDispatchFormControlChangeEvent(String& oldValue, String& newValue)
+{
+    return oldValue != newValue;
+}
+
 } // namespace WebCore