Make sure getAttribute() / setAttribute() callers use AtomicStrings

Source/core/html/HTMLButtonElement.h

 /*
  * Copyright (C) 1999 Lars Knoll (knoll@kde.org)
  *           (C) 1999 Antti Koivisto (koivisto@kde.org)
  *           (C) 2000 Dirk Mueller (mueller@kde.org)
  * Copyright (C) 2004, 2005, 2006, 2007, 2010 Apple Inc. All rights reserved.
  *
  * This library is free software; you can redistribute it and/or
  * modify it under the terms of the GNU Library General Public
  * License as published by the Free Software Foundation; either
  * version 2 of the License, or (at your option) any later version.
@@ skipping 16 matching lines @@
 #include "core/html/HTMLFormControlElement.h"
 
 namespace WebCore {
 
 class HTMLButtonElement FINAL : public HTMLFormControlElement {
 public:
     static PassRefPtr<HTMLButtonElement> create(Document&, HTMLFormElement*);
 
     void setType(const AtomicString&);
 
-    String value() const;
+    const AtomicString& value() const;
 
     virtual bool willRespondToMouseClickEvents() OVERRIDE;
 
 private:
     HTMLButtonElement(Document&, HTMLFormElement*);
 
     enum Type { SUBMIT, RESET, BUTTON };
 
     virtual const AtomicString& formControlType() const;
 
@@ skipping 24 matching lines @@
     virtual bool isOptionalFormControl() const { return true; }
     virtual bool recalcWillValidate() const;
 
     Type m_type;
     bool m_isActivatedSubmit;
 };
 
 } // namespace
 
 #endif