Introduce DEFINE_ANIMATABLE_VALUE_TYPE_CASTS to replace manual toFoo() for animatable value

Source/core/animation/AnimatableUnknown.h

 /*
  * Copyright (C) 2013 Google 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:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 53 matching lines @@
         : m_value(value)
     {
         ASSERT(m_value);
     }
     virtual AnimatableType type() const OVERRIDE { return TypeUnknown; }
     virtual bool equalTo(const AnimatableValue*) const OVERRIDE;
 
     const RefPtr<CSSValue> m_value;
 };
 
-inline const AnimatableUnknown* toAnimatableUnknown(const AnimatableValue* value)
-{
-    ASSERT_WITH_SECURITY_IMPLICATION(value && value->isUnknown());
-    return static_cast<const AnimatableUnknown*>(value);
-}
+DEFINE_ANIMATABLE_VALUE_TYPE_CASTS(AnimatableUnknown, isUnknown());
 
 inline bool AnimatableUnknown::equalTo(const AnimatableValue* value) const
 {
     const AnimatableUnknown* unknown = toAnimatableUnknown(value);
     return m_value == unknown->m_value || m_value->equals(*unknown->m_value);
 }
 
 } // namespace WebCore
 
 #endif // AnimatableUnknown_h