Remove negative z-index.

sky/engine/core/rendering/style/StyleBoxData.h

 /*
  * Copyright (C) 2000 Lars Knoll (knoll@kde.org)
  *           (C) 2000 Antti Koivisto (koivisto@kde.org)
  *           (C) 2000 Dirk Mueller (mueller@kde.org)
  * Copyright (C) 2003, 2005, 2006, 2007, 2008 Apple Inc. All rights reserved.
  * Copyright (C) 2006 Graham Dennis (graham.dennis@gmail.com)
  *
  * This library is free software; you can redistribute it and/or
  * modify it under the terms of the GNU Library General Public
  * License as published by the Free Software Foundation; either
@@ skipping 36 matching lines @@
     const Length& height() const { return m_height; }
 
     const Length& minWidth() const { return m_minWidth; }
     const Length& minHeight() const { return m_minHeight; }
 
     const Length& maxWidth() const { return m_maxWidth; }
     const Length& maxHeight() const { return m_maxHeight; }
 
     const Length& verticalAlign() const { return m_verticalAlign; }
 
-    int zIndex() const { return m_zIndex; }
+    unsigned zIndex() const { return m_zIndex; }
     bool hasAutoZIndex() const { return m_hasAutoZIndex; }
 
     EBoxSizing boxSizing() const { return static_cast<EBoxSizing>(m_boxSizing); }
     EBoxDecorationBreak boxDecorationBreak() const { return static_cast<EBoxDecorationBreak>(m_boxDecorationBreak); }
 
 private:
     friend class RenderStyle;
 
     StyleBoxData();
     StyleBoxData(const StyleBoxData&);
 
     Length m_width;
     Length m_height;
 
     Length m_minWidth;
     Length m_maxWidth;
 
     Length m_minHeight;
     Length m_maxHeight;
 
     Length m_verticalAlign;
 
-    int m_zIndex;
+    unsigned m_zIndex;
     unsigned m_hasAutoZIndex : 1;
     unsigned m_boxSizing : 1; // EBoxSizing
     unsigned m_boxDecorationBreak : 1; // EBoxDecorationBreak
 };
 
 } // namespace blink
 
 #endif  // SKY_ENGINE_CORE_RENDERING_STYLE_STYLEBOXDATA_H_