Move CSSOM classes to using MutableStylePropertySet over StylePropertySet.

Source/core/css/WebKitCSSKeyframeRule.cpp

 /*
  * 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 23 matching lines @@
 namespace WebCore {
 
 StyleKeyframe::StyleKeyframe()
 {
 }
 
 StyleKeyframe::~StyleKeyframe()
 {
 }
 
-StylePropertySet* StyleKeyframe::mutableProperties()
+MutableStylePropertySet* StyleKeyframe::mutableProperties()
 {
     if (!m_properties->isMutable())
         m_properties = m_properties->mutableCopy();
-    return m_properties.get();
+    return static_cast<MutableStylePropertySet*>(m_properties.get());
 }
     
 void StyleKeyframe::setProperties(PassRefPtr<StylePropertySet> properties)
 {
     m_properties = properties;
 }
 
 /* static */
 void StyleKeyframe::parseKeyString(const String& s, Vector<float>& keys)
 {
@@ skipping 72 matching lines @@
     info.addMember(m_propertiesCSSOMWrapper, "propertiesCSSOMWrapper");
 }
 
 void WebKitCSSKeyframeRule::reattach(StyleRuleBase*)
 {
     // No need to reattach, the underlying data is shareable on mutation.
     ASSERT_NOT_REACHED();
 }
 
 } // namespace WebCore