Don't assert when serializing -mask property.

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 578 matching lines @@
                 // BUG 49055: make sure the value was not reset in the layer check just above.
                 if ((j < size - 1 && shorthand.properties()[j + 1] == CSSPropertyBackgroundRepeatY && value)
                     || (j < size - 1 && shorthand.properties()[j + 1] == CSSPropertyWebkitMaskRepeatY && value)) {
                     RefPtr<CSSValue> yValue;
                     RefPtr<CSSValue> nextValue = values[j + 1];
                     if (nextValue->isValueList())
                         yValue = toCSSValueList(nextValue.get())->itemWithoutBoundsCheck(i);
                     else
                         yValue = nextValue;
 
+                    // background-repeat-x(y) or mask-repeat-x(y) may be like this : "initial, repeat". We can omit the implicit initial values
+                    // before starting to compare their values.
+                    if (value->isImplicitInitialValue() || yValue->isImplicitInitialValue())
+                        continue;
+
                     int xId = toCSSPrimitiveValue(value.get())->getIdent();
                     int yId = toCSSPrimitiveValue(yValue.get())->getIdent();
                     if (xId != yId) {
                         if (xId == CSSValueRepeat && yId == CSSValueNoRepeat) {
                             useRepeatXShorthand = true;
                             ++j;
                         } else if (xId == CSSValueNoRepeat && yId == CSSValueRepeat) {
                             useRepeatYShorthand = true;
                             continue;
                         }
@@ skipping 138 matching lines @@
         if (!result.isEmpty())
             result.append(' ');
         result.append(value);
     }
     if (isInitialOrInherit(commonValue))
         return commonValue;
     return result.isEmpty() ? String() : result.toString();
 }
 
 }