Implemented additive animations for length

Source/core/animation/ConstantStyleInterpolation.h

Index: Source/core/animation/ConstantStyleInterpolation.h
diff --git a/Source/core/animation/ConstantStyleInterpolation.h b/Source/core/animation/ConstantStyleInterpolation.h
index 2c72f24add61713da5607010c8dddb65d32f0326..f3c7ab57e090f6e0b5db49d207f21cdd578022dc 100644
--- a/Source/core/animation/ConstantStyleInterpolation.h
+++ b/Source/core/animation/ConstantStyleInterpolation.h
@@ -17,6 +17,11 @@ public:
         return adoptRefWillBeNoop(new ConstantStyleInterpolation(value, id));
     }
 
+    static PassRefPtrWillBeRawPtr<ConstantStyleInterpolation> create(CSSValue* value, CSSPropertyID id, AnimationEffect::CompositeOperation compositeStart, AnimationEffect::CompositeOperation compositeEnd)
+    {
+        return adoptRefWillBeNoop(new ConstantStyleInterpolation(value, id, compositeStart, compositeEnd));
+    }
+
     virtual void apply(StyleResolverState& state) const override
     {
         StyleBuilder::applyProperty(m_id, state, m_value.get());
@@ -34,6 +39,11 @@ private:
         , m_value(value)
     { }
 
+    ConstantStyleInterpolation(CSSValue* value, CSSPropertyID id, AnimationEffect::CompositeOperation compositeStart, AnimationEffect::CompositeOperation compositeEnd)
+        : StyleInterpolation(InterpolableList::create(0), InterpolableList::create(0), id, compositeStart, compositeEnd)
+        , m_value(value)
+    { }
+
     RefPtrWillBeMember<CSSValue> m_value;
 };