Add additive animation support for CSS property clip

third_party/WebKit/Source/core/animation/animatable/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 31 matching lines @@
     {
         return adoptRef(new AnimatableLengthBox(left, right, top, bottom));
     }
     const AnimatableValue* left() const { return m_left.get(); }
     const AnimatableValue* right() const { return m_right.get(); }
     const AnimatableValue* top() const { return m_top.get(); }
     const AnimatableValue* bottom() const { return m_bottom.get(); }
 
 protected:
     PassRefPtr<AnimatableValue> interpolateTo(const AnimatableValue*, double fraction) const override;
+    bool usesDefaultInterpolationWith(const AnimatableValue*) const final;
 
 private:
     AnimatableLengthBox(PassRefPtr<AnimatableValue> left, PassRefPtr<AnimatableValue> right, PassRefPtr<AnimatableValue> top, PassRefPtr<AnimatableValue> bottom)
         : m_left(left)
         , m_right(right)
         , m_top(top)
         , m_bottom(bottom)
     {
     }
     AnimatableType type() const override { return TypeLengthBox; }
     bool equalTo(const AnimatableValue*) const override;
 
     RefPtr<AnimatableValue> m_left;
     RefPtr<AnimatableValue> m_right;
     RefPtr<AnimatableValue> m_top;
     RefPtr<AnimatableValue> m_bottom;
 };
 
 DEFINE_ANIMATABLE_VALUE_TYPE_CASTS(AnimatableLengthBox, isLengthBox());
 
 } // namespace blink
 
 #endif // AnimatableLengthBox_h