Assert that convertToLength succeeds instead of returning Length(Undefined)

Source/core/css/resolver/FilterOperationResolver.cpp

Index: Source/core/css/resolver/FilterOperationResolver.cpp
diff --git a/Source/core/css/resolver/FilterOperationResolver.cpp b/Source/core/css/resolver/FilterOperationResolver.cpp
index 37706e6818df1c3ea966a647de3caab8fc96f5e6..a5afa17eff5f70ed848e45089d26c56fb2eabe45 100644
--- a/Source/core/css/resolver/FilterOperationResolver.cpp
+++ b/Source/core/css/resolver/FilterOperationResolver.cpp
@@ -463,9 +463,6 @@ bool FilterOperationResolver::createFilterOperations(CSSValue* inValue, const CS
             Length stdDeviation = Length(0, Fixed);
             if (filterValue->length() >= 1)
                 stdDeviation = firstValue->convertToLength<FixedConversion | PercentConversion>(conversionData);
-            if (stdDeviation.isUndefined())
-                return false;
-
             operations.operations().append(BlurFilterOperation::create(stdDeviation));
             break;
         }