Remove CSSOM mutability in StyleRule.

sky/engine/core/css/StyleKeyframe.h

 /*
  * Copyright (C) 2007, 2008, 2012 Apple 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 copyright
  *    notice, this list of conditions and the following disclaimer in the
@@ skipping 40 matching lines @@
     // Exposed to JavaScript.
     String keyText() const;
     void setKeyText(const String&);
 
     // Used by StyleResolver.
     const Vector<double>& keys() const;
     // Used by BisonCSSParser when constructing a new StyleKeyframe.
     void setKeys(PassOwnPtr<Vector<double> >);
 
     const StylePropertySet& properties() const { return *m_properties; }
-    MutableStylePropertySet& mutableProperties();
     void setProperties(PassRefPtr<StylePropertySet>);
 
     String cssText() const;
 
     static PassOwnPtr<Vector<double> > createKeyList(CSSParserValueList*);
 
 private:
     StyleKeyframe();
 
     RefPtr<StylePropertySet> m_properties;
     // These are both calculated lazily. Either one can be set, which invalidates the other.
     mutable String m_keyText;
     mutable OwnPtr<Vector<double> > m_keys;
 };
 
 } // namespace blink
 
 #endif  // SKY_ENGINE_CORE_CSS_STYLEKEYFRAME_H_