Assert that convertToLength succeeds instead of returning Length(Undefined)

Source/core/css/resolver/StyleBuilderConverter.cpp

Index: Source/core/css/resolver/StyleBuilderConverter.cpp
diff --git a/Source/core/css/resolver/StyleBuilderConverter.cpp b/Source/core/css/resolver/StyleBuilderConverter.cpp
index d3ffb158dce9ef8a8bf8cd81e862d021d26a9e66..3024c9654392dba5783b8fcde007f7d5901d6e63 100644
--- a/Source/core/css/resolver/StyleBuilderConverter.cpp
+++ b/Source/core/css/resolver/StyleBuilderConverter.cpp
@@ -46,7 +46,6 @@ Length StyleBuilderConverter::convertLength(StyleResolverState& state, CSSValue*
 {
     CSSPrimitiveValue* primitiveValue = toCSSPrimitiveValue(value);
     Length result = primitiveValue->convertToLength<FixedConversion | PercentConversion>(state.cssToLengthConversionData());
-    ASSERT(!result.isUndefined());
     result.setQuirk(primitiveValue->isQuirkValue());
     return result;
 }
@@ -55,7 +54,6 @@ Length StyleBuilderConverter::convertLengthOrAuto(StyleResolverState& state, CSS
 {
     CSSPrimitiveValue* primitiveValue = toCSSPrimitiveValue(value);
     Length result = primitiveValue->convertToLength<FixedConversion | PercentConversion | AutoConversion>(state.cssToLengthConversionData());
-    ASSERT(!result.isUndefined());
     result.setQuirk(primitiveValue->isQuirkValue());
     return result;
 }