Fix 50% flip handling of CSS Animations using CSS wide keywords

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 43 matching lines @@
         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; }
 
+        PassRefPtrWillBeRawPtr<Interpolation> createLegacyStyleInterpolation(CSSPropertyID, Keyframe::PropertySpecificKeyframe& end, Element*, const ComputedStyle* baseStyle) const;
         static bool createInterpolationsFromCSSValues(CSSPropertyID, CSSValue* fromCSSValue, CSSValue* toCSSValue, Element*, OwnPtrWillBeRawPtr<WillBeHeapVector<RefPtrWillBeMember<Interpolation>>>& interpolations);
 
         void populateAnimatableValueCaches(CSSPropertyID, Keyframe::PropertySpecificKeyframe&, Element*, CSSValue& fromCSSValue, CSSValue& toCSSValue) const;
 
         RefPtrWillBeMember<CSSValue> m_value;
         mutable RefPtrWillBeMember<AnimatableValue> m_animatableValueCache;
     };
 
     class SVGPropertySpecificKeyframe : public Keyframe::PropertySpecificKeyframe {
     public:
@@ skipping 37 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