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

Source/core/html/HTMLDocument.cpp

 /*
  * Copyright (C) 1999 Lars Knoll (knoll@kde.org)
  *           (C) 1999 Antti Koivisto (koivisto@kde.org)
  * Copyright (C) 2003, 2004, 2005, 2006, 2007, 2008 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 61 matching lines @@
     : Document(initializer, HTMLDocumentClass | extendedDocumentClasses)
 {
     ScriptWrappable::init(this);
     clearXMLVersion();
 }
 
 HTMLDocument::~HTMLDocument()
 {
 }
 
-String HTMLDocument::dir()
+const AtomicString& HTMLDocument::dir()
 {
     HTMLElement* b = body();
     if (!b)
-        return String();
+        return nullAtom;
     return b->getAttribute(dirAttr);
 }
 
-void HTMLDocument::setDir(const String& value)
+void HTMLDocument::setDir(const AtomicString& value)
 {
     HTMLElement* b = body();
     if (b)
         b->setAttribute(dirAttr, value);
 }
 
 String HTMLDocument::designMode() const
 {
     return inDesignMode() ? "on" : "off";
 }
@@ skipping 239 matching lines @@
 void HTMLDocument::writeln(DOMWindow* activeWindow, const Vector<String>& text)
 {
     ASSERT(activeWindow);
     StringBuilder builder;
     for (size_t i = 0; i < text.size(); ++i)
         builder.append(text[i]);
     writeln(builder.toString(), activeWindow->document());
 }
 
 }