Implemented additive animations for length

Source/core/animation/LengthStyleInterpolation.h

Index: Source/core/animation/LengthStyleInterpolation.h
diff --git a/Source/core/animation/LengthStyleInterpolation.h b/Source/core/animation/LengthStyleInterpolation.h
index 35401806cf657de17e86260261382a417064a3dd..17464267170e1374f8f7582b6564ed81a7261d01 100644
--- a/Source/core/animation/LengthStyleInterpolation.h
+++ b/Source/core/animation/LengthStyleInterpolation.h
@@ -21,9 +21,15 @@ public:
         return adoptRefWillBeNoop(new LengthStyleInterpolation(toInterpolableValue(start), toInterpolableValue(end), id, range));
     }
 
+    static PassRefPtrWillBeRawPtr<LengthStyleInterpolation> create(const CSSValue& start, const CSSValue& end, CSSPropertyID id, InterpolationRange range, AnimationEffect::CompositeOperation compositeStart, AnimationEffect::CompositeOperation compositeEnd)
+    {
+        return adoptRefWillBeNoop(new LengthStyleInterpolation(toInterpolableValue(start), toInterpolableValue(end), id, range, compositeStart, compositeEnd));
+    }
+
     static bool canCreateFrom(const CSSValue&);
 
     virtual void apply(StyleResolverState&) const override;
+    virtual void apply(StyleResolverState&, const InterpolableValue&) const override;
 
     virtual void trace(Visitor*) override;
 
@@ -36,6 +42,11 @@ private:
         , m_range(range)
         { }
 
+    LengthStyleInterpolation(PassOwnPtrWillBeRawPtr<InterpolableValue> start, PassOwnPtrWillBeRawPtr<InterpolableValue> end, CSSPropertyID id,  InterpolationRange range, AnimationEffect::CompositeOperation compositeStart, AnimationEffect::CompositeOperation compositeEnd)
+        : StyleInterpolation(start, end, id, compositeStart, compositeEnd)
+        , m_range(range)
+        { }
+
     InterpolationRange m_range;
 };