Add [TreatReturnedNullStringAs] support for ScalarValueString

Source/core/testing/TypeConversions.idl

Index: Source/core/testing/TypeConversions.idl
diff --git a/Source/core/testing/TypeConversions.idl b/Source/core/testing/TypeConversions.idl
index 79e1fc6e3f9479fedb6add5dd465bd8f92edb200..86dc3284b932e0fcdad6007e89fbeb4b69544abb 100644
--- a/Source/core/testing/TypeConversions.idl
+++ b/Source/core/testing/TypeConversions.idl
@@ -55,6 +55,10 @@
     [ImplementedAs=setTestByteString] void setTestByteStringDefaultNull(optional ByteString byteString = null);
 
     attribute ScalarValueString testScalarValueString;
+    [ImplementedAs=testScalarValueString, TreatReturnedNullStringAs=Null] attribute ScalarValueString testScalarValueStringTreatReturnedNullStringAsNullAttribute;
+    [ImplementedAs=testScalarValueString, TreatReturnedNullStringAs=Undefined] attribute ScalarValueString testScalarValueStringTreatReturnedNullStringAsUndefinedAttribute;
+    [ImplementedAs=testScalarValueString, TreatReturnedNullStringAs=Null] ScalarValueString getTestScalarValueStringTreatReturnedNullStringAsNullMethod();
+    [ImplementedAs=testScalarValueString, TreatReturnedNullStringAs=Undefined] ScalarValueString getTestScalarValueStringTreatReturnedNullStringAsUndefinedMethod();
     void setTestScalarValueString(ScalarValueString scalarValueString);
     [ImplementedAs=setTestScalarValueString] void setTestScalarValueStringDefaultNull(optional ScalarValueString scalarValueString = null);
 };