Move create() methods from StylePropertySet to MutableStylePropertySet.

Source/core/dom/Attr.cpp

 /*
  * Copyright (C) 1999 Lars Knoll (knoll@kde.org)
  *           (C) 1999 Antti Koivisto (koivisto@kde.org)
  *           (C) 2001 Peter Kelly (pmk@post.com)
  *           (C) 2001 Dirk Mueller (mueller@kde.org)
  * Copyright (C) 2004, 2005, 2006, 2007, 2009, 2010, 2012 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
@@ skipping 172 matching lines @@
 bool Attr::isId() const
 {
     return qualifiedName().matches(document()->idAttributeName());
 }
 
 CSSStyleDeclaration* Attr::style()
 {
     // This function only exists to support the Obj-C bindings.
     if (!m_element || !m_element->isStyledElement())
         return 0;
-    m_style = StylePropertySet::create();
+    m_style = MutableStylePropertySet::create();
     static_cast<StyledElement*>(m_element)->collectStyleForPresentationAttribute(qualifiedName(), value(), static_cast<MutableStylePropertySet*>(m_style.get()));
     return m_style->ensureCSSStyleDeclaration();
 }
 
 const AtomicString& Attr::value() const
 {
     if (m_element)
         return m_element->getAttribute(qualifiedName());
     return m_standaloneValue;
 }
@@ skipping 14 matching lines @@
 }
 
 void Attr::attachToElement(Element* element)
 {
     ASSERT(!m_element);
     m_element = element;
     m_standaloneValue = nullAtom;
 }
 
 }