Add foundation for removing AnimatableValues from StyleInterpolation

Source/core/animation/StringKeyframe.h

Index: Source/core/animation/StringKeyframe.h
diff --git a/Source/core/animation/StringKeyframe.h b/Source/core/animation/StringKeyframe.h
index 839fe75da8166b60e590fee3985754a2b18195b9..58941eeb6d8183d4f6c90e312812afa84695c7e0 100644
--- a/Source/core/animation/StringKeyframe.h
+++ b/Source/core/animation/StringKeyframe.h
@@ -49,15 +49,15 @@ public:
         virtual const PassRefPtrWillBeRawPtr<AnimatableValue> getAnimatableValue() const override final { return m_animatableValueCache.get(); }
         void setAnimatableValue(PassRefPtrWillBeRawPtr<AnimatableValue> value) { m_animatableValueCache = value; }
 
-        virtual PassOwnPtrWillBeRawPtr<Keyframe::PropertySpecificKeyframe> neutralKeyframe(double offset, PassRefPtr<TimingFunction> easing) const override final;
-        virtual PassRefPtrWillBeRawPtr<Interpolation> maybeCreateInterpolation(PropertyHandle, Keyframe::PropertySpecificKeyframe& end, Element*, const ComputedStyle* baseStyle) const override final;
+        virtual PassRefPtrWillBeRawPtr<Keyframe::PropertySpecificKeyframe> neutralKeyframe(double offset, PassRefPtr<TimingFunction> easing) const override final;
+        virtual PassRefPtrWillBeRawPtr<Interpolation> maybeCreateInterpolation(PropertyHandle, Keyframe::PropertySpecificKeyframe& end, Element*, const ComputedStyle* baseStyle) override final;
 
         DECLARE_VIRTUAL_TRACE();
 
     private:
         CSSPropertySpecificKeyframe(double offset, PassRefPtr<TimingFunction> easing, CSSValue*);
 
-        virtual PassOwnPtrWillBeRawPtr<Keyframe::PropertySpecificKeyframe> cloneWithOffset(double offset) const;
+        virtual PassRefPtrWillBeRawPtr<Keyframe::PropertySpecificKeyframe> cloneWithOffset(double offset) const;
         virtual bool isCSSPropertySpecificKeyframe() const override { return true; }
 
         static bool createInterpolationsFromCSSValues(CSSPropertyID, CSSValue* fromCSSValue, CSSValue* toCSSValue, Element*, OwnPtrWillBeRawPtr<WillBeHeapVector<RefPtrWillBeMember<Interpolation>>>& interpolations);
@@ -74,12 +74,12 @@ public:
 
         const String& value() const { return m_value; }
 
-        virtual PassOwnPtrWillBeRawPtr<PropertySpecificKeyframe> cloneWithOffset(double offset) const override final;
+        virtual PassRefPtrWillBeRawPtr<PropertySpecificKeyframe> cloneWithOffset(double offset) const override final;
 
         virtual const PassRefPtrWillBeRawPtr<AnimatableValue> getAnimatableValue() const override final { return nullptr; }
 
-        virtual PassOwnPtrWillBeRawPtr<PropertySpecificKeyframe> neutralKeyframe(double offset, PassRefPtr<TimingFunction> easing) const override final;
-        virtual PassRefPtrWillBeRawPtr<Interpolation> maybeCreateInterpolation(PropertyHandle, Keyframe::PropertySpecificKeyframe& end, Element*, const ComputedStyle* baseStyle) const override final;
+        virtual PassRefPtrWillBeRawPtr<PropertySpecificKeyframe> neutralKeyframe(double offset, PassRefPtr<TimingFunction> easing) const override final;
+        virtual PassRefPtrWillBeRawPtr<Interpolation> maybeCreateInterpolation(PropertyHandle, Keyframe::PropertySpecificKeyframe& end, Element*, const ComputedStyle* baseStyle) override final;
 
         DECLARE_VIRTUAL_TRACE();
 
@@ -99,7 +99,7 @@ private:
     StringKeyframe(const StringKeyframe& copyFrom);
 
     virtual PassRefPtrWillBeRawPtr<Keyframe> clone() const override;
-    virtual PassOwnPtrWillBeRawPtr<Keyframe::PropertySpecificKeyframe> createPropertySpecificKeyframe(PropertyHandle) const override;
+    virtual PassRefPtrWillBeRawPtr<Keyframe::PropertySpecificKeyframe> createPropertySpecificKeyframe(PropertyHandle) const override;
 
     virtual bool isStringKeyframe() const override { return true; }