Assert that convertToLength succeeds instead of returning Length(Undefined)

Source/core/css/CSSCalculationValue.cpp

Index: Source/core/css/CSSCalculationValue.cpp
diff --git a/Source/core/css/CSSCalculationValue.cpp b/Source/core/css/CSSCalculationValue.cpp
index e27cba3a66db7d1722794fe52a7a0ceb7ee51532..673da779775dd4e267ee5ece84e7ce6e673ad277 100644
--- a/Source/core/css/CSSCalculationValue.cpp
+++ b/Source/core/css/CSSCalculationValue.cpp
@@ -231,12 +231,8 @@ public:
         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: