Add support for z-index: auto to AnimatableValues

Source/core/animation/css/CSSAnimatableValueFactory.cpp

 /*
  * Copyright (C) 2013 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
  * met:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 544 matching lines @@
         return createFromLength(style.svgStyle().x(), style);
     case CSSPropertyY:
         return createFromLength(style.svgStyle().y(), style);
     case CSSPropertyR:
         return createFromLength(style.svgStyle().r(), style);
     case CSSPropertyRx:
         return createFromLength(style.svgStyle().rx(), style);
     case CSSPropertyRy:
         return createFromLength(style.svgStyle().ry(), style);
     case CSSPropertyZIndex:
+        if (style.hasAutoZIndex())
+            return AnimatableUnknown::create(CSSValueAuto);
         return createFromDouble(style.zIndex());
     default:
         ASSERT_NOT_REACHED();
         return nullptr;
     }
 }
 
 } // namespace blink