Implemented additive animations for length

Source/core/animation/ConstantStyleInterpolation.h

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef ConstantStyleInterpolation_h
 #define ConstantStyleInterpolation_h
 
 #include "core/animation/StyleInterpolation.h"
 #include "core/css/resolver/StyleBuilder.h"
 
 namespace blink {
 
 class ConstantStyleInterpolation : public StyleInterpolation {
 public:
     static PassRefPtrWillBeRawPtr<ConstantStyleInterpolation> create(CSSValue* value, CSSPropertyID id)
     {
         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());
     }
 
     virtual void trace(Visitor* visitor) override
     {
         StyleInterpolation::trace(visitor);
         visitor->trace(m_value);
     }
 
 private:
     ConstantStyleInterpolation(CSSValue* value, CSSPropertyID id)
         : StyleInterpolation(InterpolableList::create(0), InterpolableList::create(0), id)
         , 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;
 };
 
 }
 
 #endif