Add support for z-index: auto to AnimatableValues

Source/core/animation/css/CSSPropertyEquality.cpp

Index: Source/core/animation/css/CSSPropertyEquality.cpp
diff --git a/Source/core/animation/css/CSSPropertyEquality.cpp b/Source/core/animation/css/CSSPropertyEquality.cpp
index 2425a13cb5aee0c272c0cad6dc71f96f453b08f8..09b7829a88c15b86d53937b96bbea9696e3a1377 100644
--- a/Source/core/animation/css/CSSPropertyEquality.cpp
+++ b/Source/core/animation/css/CSSPropertyEquality.cpp
@@ -322,7 +322,7 @@ bool CSSPropertyEquality::propertiesEqual(CSSPropertyID prop, const ComputedStyl
     case CSSPropertyRy:
         return a.svgStyle().ry() == b.svgStyle().ry();
     case CSSPropertyZIndex:
-        return a.zIndex() == b.zIndex();
+        return a.hasAutoZIndex() == b.hasAutoZIndex() && (a.hasAutoZIndex() || a.zIndex() == b.zIndex());
     default:
         ASSERT_NOT_REACHED();
         return true;