Remove negative z-index.

sky/engine/core/css/resolver/AnimatedStyleBuilder.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 491 matching lines @@
     case CSSPropertyWidth:
         style->setWidth(animatableValueToLength(value, state, ValueRangeNonNegative));
         return;
     case CSSPropertyWordSpacing:
         style->setWordSpacing(clampTo<float>(toAnimatableDouble(value)->toDouble()));
         return;
     case CSSPropertyVerticalAlign:
         style->setVerticalAlignLength(animatableValueToLength(value, state));
         return;
     case CSSPropertyZIndex:
-        style->setZIndex(animatableValueRoundClampTo<int>(value));
+        style->setZIndex(animatableValueRoundClampTo<unsigned>(value));
         return;
     default:
         ASSERT_NOT_REACHED();
     }
 }
 
 } // namespace blink