Handle null in value list in getLayeredShorthandValue

Source/core/css/StylePropertySerializer.cpp

 /*
  * (C) 1999-2003 Lars Knoll (knoll@kde.org)
  * Copyright (C) 2004, 2005, 2006, 2007, 2008, 2009, 2010, 2012 Apple Inc. All rights reserved.
  * Copyright (C) 2011 Research In Motion Limited. All rights reserved.
  * Copyright (C) 2013 Intel Corporation. All rights reserved.
  *
  * 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
  * version 2 of the License, or (at your option) any later version.
@@ skipping 383 matching lines @@
     switch (propertyID) {
     case CSSPropertyAnimation:
         return getLayeredShorthandValue(animationShorthand(), true);
     case CSSPropertyBorderSpacing:
         return borderSpacingValue(borderSpacingShorthand());
     case CSSPropertyBackgroundPosition:
         return getLayeredShorthandValue(backgroundPositionShorthand());
     case CSSPropertyBackgroundRepeat:
         return backgroundRepeatPropertyValue();
     case CSSPropertyBackground:
-        return getLayeredShorthandValue(backgroundShorthand());
+        return getLayeredShorthandValue(backgroundShorthand(), true);
     case CSSPropertyBorder:
         return borderPropertyValue(OmitUncommonValues);
     case CSSPropertyBorderTop:
         return getShorthandValue(borderTopShorthand());
     case CSSPropertyBorderRight:
         return getShorthandValue(borderRightShorthand());
     case CSSPropertyBorderBottom:
         return getShorthandValue(borderBottomShorthand());
     case CSSPropertyBorderLeft:
         return getShorthandValue(borderLeftShorthand());
@@ skipping 595 matching lines @@
             isInitialValue = false;
         if (!value->isInheritedValue())
             isInheritedValue = false;
         if (isImportant != m_propertySet.propertyIsImportant(shorthand.properties()[i]))
             return false;
     }
     return isInitialValue || isInheritedValue;
 }
 
 }