Revert "Web Animations: Introduce String based KeyframeEffectModel"

Source/core/animation/Interpolation.h

Index: Source/core/animation/Interpolation.h
diff --git a/Source/core/animation/Interpolation.h b/Source/core/animation/Interpolation.h
index 8bc64e84eed37ceed294d0c9198d1e9a5724769e..28b03b517a2c47f492c604e67799156eb4b23b71 100644
--- a/Source/core/animation/Interpolation.h
+++ b/Source/core/animation/Interpolation.h
@@ -44,6 +44,7 @@ private:
 
     friend class AnimationInterpolableValueTest;
     friend class AnimationInterpolationEffectTest;
+
 };
 
 class StyleInterpolation : public Interpolation {
@@ -82,7 +83,7 @@ public:
     virtual void apply(StyleResolverState&) const;
 
     virtual bool isLegacyStyleInterpolation() const OVERRIDE FINAL { return true; }
-    PassRefPtr<AnimatableValue> currentValue() const
+    AnimatableValue* currentValue() const
     {
         InterpolableAnimatableValue* value = static_cast<InterpolableAnimatableValue*>(m_cachedValue.get());
         return value->value();