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

Source/core/html/HTMLImageElement.h

 /*
  * Copyright (C) 1999 Lars Knoll (knoll@kde.org)
  *           (C) 1999 Antti Koivisto (koivisto@kde.org)
  * Copyright (C) 2004, 2008, 2010 Apple Inc. All rights reserved.
  * Copyright (C) 2010 Google 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 31 matching lines @@
     virtual ~HTMLImageElement();
 
     int width(bool ignorePendingStylesheets = false);
     int height(bool ignorePendingStylesheets = false);
 
     int naturalWidth() const;
     int naturalHeight() const;
 
     bool isServerMap() const;
 
-    String altText() const;
+    const AtomicString& altText() const;
 
     CompositeOperator compositeOperator() const { return m_compositeOperator; }
 
     ImageResource* cachedImage() const { return m_imageLoader.image(); }
     void setImageResource(ImageResource* i) { m_imageLoader.setImage(i); };
 
     void setLoadManually(bool loadManually) { m_imageLoader.setLoadManually(loadManually); }
 
     const AtomicString& alt() const;
 
@@ skipping 53 matching lines @@
     CompositeOperator m_compositeOperator;
     AtomicString m_bestFitImageURL;
     float m_imageDevicePixelRatio;
 };
 
 DEFINE_NODE_TYPE_CASTS(HTMLImageElement, hasTagName(HTMLNames::imgTag));
 
 } //namespace
 
 #endif