Remove CSSOM mutability in StyleRule.

sky/engine/core/css/StyleKeyframe.cpp

Index: sky/engine/core/css/StyleKeyframe.cpp
diff --git a/sky/engine/core/css/StyleKeyframe.cpp b/sky/engine/core/css/StyleKeyframe.cpp
index 78d2cfbae86497d96fa1b4b4ccbb576415fa30a2..94ee838623555adb05e0ca1a76cd0df076d92651 100644
--- a/sky/engine/core/css/StyleKeyframe.cpp
+++ b/sky/engine/core/css/StyleKeyframe.cpp
@@ -89,13 +89,6 @@ void StyleKeyframe::setKeys(PassOwnPtr<Vector<double> > keys)
     ASSERT(m_keyText.isNull());
 }
 
-MutableStylePropertySet& StyleKeyframe::mutableProperties()
-{
-    if (!m_properties->isMutable())
-        m_properties = m_properties->mutableCopy();
-    return *toMutableStylePropertySet(m_properties.get());
-}
-
 void StyleKeyframe::setProperties(PassRefPtr<StylePropertySet> properties)
 {
     ASSERT(properties);