Serialize zIndex as an integer

Source/core/animation/DoubleStyleInterpolation.h

Index: Source/core/animation/DoubleStyleInterpolation.h
diff --git a/Source/core/animation/DoubleStyleInterpolation.h b/Source/core/animation/DoubleStyleInterpolation.h
index 2a493bcc17fa45670e6b8b3f1bfb53f20ce1a435..688513c68115f03a42f8a479d8bb6eb28446ae87 100644
--- a/Source/core/animation/DoubleStyleInterpolation.h
+++ b/Source/core/animation/DoubleStyleInterpolation.h
@@ -17,7 +17,7 @@ public:
 
     static PassRefPtrWillBeRawPtr<DoubleStyleInterpolation> create(const CSSValue& start, const CSSValue& end, CSSPropertyID id, CSSPrimitiveValue::UnitType type, InterpolationRange clamp)
     {
-        return adoptRefWillBeNoop(new DoubleStyleInterpolation(doubleToInterpolableValue(start), doubleToInterpolableValue(end), id, type == CSSPrimitiveValue::CSS_NUMBER, clamp, false));
+        return adoptRefWillBeNoop(new DoubleStyleInterpolation(doubleToInterpolableValue(start), doubleToInterpolableValue(end), id, type == CSSPrimitiveValue::CSS_NUMBER || type == CSSPrimitiveValue::CSS_INTEGER, clamp, false));
     }
 
     static PassRefPtrWillBeRawPtr<DoubleStyleInterpolation> maybeCreateFromMotionRotation(const CSSValue& start, const CSSValue& end, CSSPropertyID);