Implemented additive animations for length

Source/core/animation/StyleInterpolation.h

Index: Source/core/animation/StyleInterpolation.h
diff --git a/Source/core/animation/StyleInterpolation.h b/Source/core/animation/StyleInterpolation.h
index ee5fb221a04e5f1ea29c68b7c5f5f2270edd2215..33bcd20bb0c699c43960a30bf08168be1433f732 100644
--- a/Source/core/animation/StyleInterpolation.h
+++ b/Source/core/animation/StyleInterpolation.h
@@ -28,6 +28,9 @@ public:
     // (3) a custom value that is inserted directly into the StyleResolverState.
     virtual void apply(StyleResolverState&) const = 0;
 
+    // FIXME: Make abstract
+    virtual void apply(StyleResolverState& state, const InterpolableValue&) const { apply(state); }
+
     virtual bool isStyleInterpolation() const override final { return true; }
 
     CSSPropertyID id() const { return m_id; }
@@ -45,6 +48,12 @@ protected:
         , m_id(id)
     {
     }
+
+    StyleInterpolation(PassOwnPtrWillBeRawPtr<InterpolableValue> start, PassOwnPtrWillBeRawPtr<InterpolableValue> end, CSSPropertyID id, AnimationEffect::CompositeOperation compositeStart, AnimationEffect::CompositeOperation compositeEnd)
+        : Interpolation(start, end, compositeStart, compositeEnd)
+        , m_id(id)
+    {
+    }
 };
 
 DEFINE_TYPE_CASTS(StyleInterpolation, Interpolation, value, value->isStyleInterpolation(), value.isStyleInterpolation());