Move create() methods from StylePropertySet to MutableStylePropertySet.

Source/core/css/DOMWindowCSS.cpp

 /*
  * Copyright (C) 2012 Motorola Mobility Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
  * met:
  *
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2. Redistributions in binary form must reproduce the above
@@ skipping 56 matching lines @@
 
     // CSSParser::parseValue() won't work correctly if !important is present,
     // so just get rid of it. It doesn't matter to supports() if it's actually
     // there or not, provided how it's specified in the value is correct.
     String normalizedValue = value.stripWhiteSpace().simplifyWhiteSpace();
     normalizedValue = valueWithoutImportant(normalizedValue);
 
     if (normalizedValue.isEmpty())
         return false;
 
-    RefPtr<StylePropertySet> dummyStyle = StylePropertySet::create();
+    RefPtr<MutableStylePropertySet> dummyStyle = MutableStylePropertySet::create();
     return CSSParser::parseValue(dummyStyle.get(), propertyID, normalizedValue, false, CSSStrictMode, 0);
 }
 
 bool DOMWindowCSS::supports(const String& conditionText) const
 {
     CSSParserContext context(CSSStrictMode);
     CSSParser parser(context);
     return parser.parseSupportsCondition(conditionText);
 }
 
 }