Implement left property animation on InvalidatableStyleInterpolation

Source/core/animation/StringKeyframe.h

 // Copyright 2014 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 StringKeyframe_h
 #define StringKeyframe_h
 
 #include "core/animation/Keyframe.h"
 #include "core/css/StylePropertySet.h"
 
@@ skipping 31 matching lines @@
     class CSSPropertySpecificKeyframe : public Keyframe::PropertySpecificKeyframe {
     public:
         CSSPropertySpecificKeyframe(double offset, PassRefPtr<TimingFunction> easing, CSSValue*, EffectModel::CompositeOperation);
 
         CSSValue* value() const { return m_value.get(); }
 
         virtual void populateAnimatableValue(CSSPropertyID, Element&, const ComputedStyle* baseStyle) const;
         const PassRefPtrWillBeRawPtr<AnimatableValue> getAnimatableValue() const final { return m_animatableValueCache.get(); }
         void setAnimatableValue(PassRefPtrWillBeRawPtr<AnimatableValue> value) { m_animatableValueCache = value; }
 
+        bool isNeutral() const final { return !m_value; }
         PassOwnPtrWillBeRawPtr<Keyframe::PropertySpecificKeyframe> neutralKeyframe(double offset, PassRefPtr<TimingFunction> easing) const final;
         PassRefPtrWillBeRawPtr<Interpolation> maybeCreateInterpolation(PropertyHandle, Keyframe::PropertySpecificKeyframe& end, Element*, const ComputedStyle* baseStyle) const final;
 
         DECLARE_VIRTUAL_TRACE();
 
     private:
         CSSPropertySpecificKeyframe(double offset, PassRefPtr<TimingFunction> easing, CSSValue*);
 
         virtual PassOwnPtrWillBeRawPtr<Keyframe::PropertySpecificKeyframe> cloneWithOffset(double offset) const;
         bool isCSSPropertySpecificKeyframe() const override { return true; }
@@ skipping 48 matching lines @@
 using CSSPropertySpecificKeyframe = StringKeyframe::CSSPropertySpecificKeyframe;
 using SVGPropertySpecificKeyframe = StringKeyframe::SVGPropertySpecificKeyframe;
 
 DEFINE_TYPE_CASTS(StringKeyframe, Keyframe, value, value->isStringKeyframe(), value.isStringKeyframe());
 DEFINE_TYPE_CASTS(CSSPropertySpecificKeyframe, Keyframe::PropertySpecificKeyframe, value, value->isCSSPropertySpecificKeyframe(), value.isCSSPropertySpecificKeyframe());
 DEFINE_TYPE_CASTS(SVGPropertySpecificKeyframe, Keyframe::PropertySpecificKeyframe, value, value->isSVGPropertySpecificKeyframe(), value.isSVGPropertySpecificKeyframe());
 
 }
 
 #endif