Add support for z-index: auto to AnimatableValues

Source/core/animation/css/CSSPropertyEquality.cpp

 // 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.
 
 #include "config.h"
 #include "core/animation/css/CSSPropertyEquality.h"
 
 #include "core/animation/css/CSSAnimations.h"
 #include "core/style/DataEquivalency.h"
 #include "core/style/ComputedStyle.h"
@@ skipping 304 matching lines @@
         return a.svgStyle().x() == b.svgStyle().x();
     case CSSPropertyY:
         return a.svgStyle().y() == b.svgStyle().y();
     case CSSPropertyR:
         return a.svgStyle().r() == b.svgStyle().r();
     case CSSPropertyRx:
         return a.svgStyle().rx() == b.svgStyle().rx();
     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;
     }
 }
 
 }