Animation: Fix loss of type information when interpolating value of 0

Source/core/animation/LengthPairStyleInterpolation.h

Index: Source/core/animation/LengthPairStyleInterpolation.h
diff --git a/Source/core/animation/LengthPairStyleInterpolation.h b/Source/core/animation/LengthPairStyleInterpolation.h
index 18e2a280832df47f1dc6dea9bbd1ac8c7cc2dd04..a9e30f3b283d3c7c2895ec861b860eef81c286f5 100644
--- a/Source/core/animation/LengthPairStyleInterpolation.h
+++ b/Source/core/animation/LengthPairStyleInterpolation.h
@@ -12,6 +12,7 @@ namespace blink {
 
 class LengthPairStyleInterpolation : public StyleInterpolation {
 public:
+
     static PassRefPtrWillBeRawPtr<LengthPairStyleInterpolation> create(const CSSValue& start, const CSSValue& end, CSSPropertyID id, ValueRange range)
     {
         return adoptRefWillBeNoop(new LengthPairStyleInterpolation(lengthPairToInterpolableValue(start), lengthPairToInterpolableValue(end), id, range));
@@ -20,6 +21,8 @@ public:
     static bool canCreateFrom(const CSSValue&);
 
     virtual void apply(StyleResolverState&) const override;
+    virtual void trace(Visitor*) override;
+
 
 private:
     LengthPairStyleInterpolation(PassOwnPtrWillBeRawPtr<InterpolableValue> start, PassOwnPtrWillBeRawPtr<InterpolableValue> end, CSSPropertyID id, ValueRange range)
@@ -32,7 +35,7 @@ private:
 
     ValueRange m_range;
 
-    friend class LengthPairStyleInterpolationTest;
+    friend class AnimationLengthPairStyleInterpolationTest;
 };
 
 } // namespace blink