Move create() methods from StylePropertySet to MutableStylePropertySet.

Source/core/dom/Attr.cpp

Index: Source/core/dom/Attr.cpp
diff --git a/Source/core/dom/Attr.cpp b/Source/core/dom/Attr.cpp
index 366e507940ad7efa51b151f808aa779b982f0d2d..20101680740a38b33695e1aae371291c9c9eccfc 100644
--- a/Source/core/dom/Attr.cpp
+++ b/Source/core/dom/Attr.cpp
@@ -190,7 +190,7 @@ 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();
 }