Introduce DEFINE_ANIMATABLE_VALUE_TYPE_CASTS to replace manual toFoo() for animatable value

Source/core/animation/AnimatableLengthBox.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 50 matching lines @@
     }
     virtual AnimatableType type() const OVERRIDE { return TypeLengthBox; }
     virtual bool equalTo(const AnimatableValue*) const OVERRIDE;
 
     RefPtr<AnimatableValue> m_left;
     RefPtr<AnimatableValue> m_right;
     RefPtr<AnimatableValue> m_top;
     RefPtr<AnimatableValue> m_bottom;
 };
 
-inline const AnimatableLengthBox* toAnimatableLengthBox(const AnimatableValue* value)
-{
-    ASSERT_WITH_SECURITY_IMPLICATION(value && value->isLengthBox());
-    return static_cast<const AnimatableLengthBox*>(value);
-}
+DEFINE_ANIMATABLE_VALUE_TYPE_CASTS(AnimatableLengthBox, isLengthBox());
 
 } // namespace WebCore
 
 #endif // AnimatableLengthBox_h