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

Source/core/rendering/HitTestResult.cpp

 /*
  * Copyright (C) 2006, 2008, 2011 Apple Inc. All rights reserved.
  * Copyright (C) 2012 Nokia Corporation and/or its subsidiary(-ies)
  *
  * 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.
  *
  * This library is distributed in the hope that it will be useful,
@@ skipping 230 matching lines @@
             if (!title.isEmpty()) {
                 if (RenderObject* renderer = titleNode->renderer())
                     dir = renderer->style()->direction();
                 return title;
             }
         }
     }
     return String();
 }
 
-String HitTestResult::altDisplayString() const
+const AtomicString& HitTestResult::altDisplayString() const
 {
     if (!m_innerNonSharedNode)
-        return String();
+        return nullAtom;
 
     if (m_innerNonSharedNode->hasTagName(imgTag)) {
         HTMLImageElement* image = toHTMLImageElement(m_innerNonSharedNode);
         return image->getAttribute(altAttr);
     }
 
     if (m_innerNonSharedNode->hasTagName(inputTag)) {
         HTMLInputElement* input = toHTMLInputElement(m_innerNonSharedNode);
         return input->alt();
     }
 
-    return String();
+    return nullAtom;
 }
 
 Image* HitTestResult::image() const
 {
     if (!m_innerNonSharedNode)
         return 0;
 
     RenderObject* renderer = m_innerNonSharedNode->renderer();
     if (renderer && renderer->isImage()) {
         RenderImage* image = toRenderImage(renderer);
@@ skipping 230 matching lines @@
 {
     for (Node* node = m_innerNode.get(); node; node = NodeRenderingTraversal::parent(node)) {
         if (node->isElementNode())
             return toElement(node);
     }
 
     return 0;
 }
 
 } // namespace WebCore