Assert that convertToLength succeeds instead of returning Length(Undefined)

Source/core/css/CSSCalculationValue.cpp

 /*
  * Copyright (C) 2011, 2012 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 213 matching lines @@
     }
 
     virtual PassOwnPtr<CalcExpressionNode> toCalcValue(const CSSToLengthConversionData& conversionData) const
     {
         switch (m_category) {
         case CalcNumber:
             return adoptPtr(new CalcExpressionNumber(m_value->getFloatValue()));
         case CalcLength:
             return adoptPtr(new CalcExpressionLength(Length(m_value->computeLength<float>(conversionData), WebCore::Fixed)));
         case CalcPercent:
-        case CalcPercentLength: {
-            CSSPrimitiveValue* primitiveValue = m_value.get();
-            return adoptPtr(new CalcExpressionLength(primitiveValue
-                ? primitiveValue->convertToLength<FixedConversion | PercentConversion>(conversionData)
-                : Length(Undefined)));
-        }
+        case CalcPercentLength:
+            return adoptPtr(new CalcExpressionLength(m_value->convertToLength<FixedConversion | PercentConversion>(conversionData)));
         // Only types that could be part of a Length expression can be converted
         // to a CalcExpressionNode. CalcPercentNumber makes no sense as a Length.
         case CalcPercentNumber:
         case CalcVariable:
         case CalcOther:
             ASSERT_NOT_REACHED();
         }
         return nullptr;
     }
 
@@ skipping 531 matching lines @@
 
     return expression ? adoptRef(new CSSCalcValue(expression, range)) : 0;
 }
 
 PassRefPtr<CSSCalcValue> CSSCalcValue::create(PassRefPtr<CSSCalcExpressionNode> expression, ValueRange range)
 {
     return adoptRef(new CSSCalcValue(expression, range));
 }
 
 } // namespace WebCore