Add [TreatReturnedNullStringAs] support for ScalarValueString

Source/bindings/tests/idls/TestObject.idl

Index: Source/bindings/tests/idls/TestObject.idl
diff --git a/Source/bindings/tests/idls/TestObject.idl b/Source/bindings/tests/idls/TestObject.idl
index 73085953233b495e79be5afd7daf4ca7b876cf15..8328c86d78466d1c529287582952b0d98f12bb25 100644
--- a/Source/bindings/tests/idls/TestObject.idl
+++ b/Source/bindings/tests/idls/TestObject.idl
@@ -241,6 +241,8 @@ interface TestObject {
     [TreatReturnedNullStringAs=Undefined] attribute DOMString treatReturnedNullStringAsUndefinedStringAttribute;
     [TreatReturnedNullStringAs=Null] attribute ByteString treatReturnedNullStringAsNullByteStringAttribute;
     [TreatReturnedNullStringAs=Undefined] attribute ByteString treatReturnedNullStringAsUndefinedByteStringAttribute;
+    [TreatReturnedNullStringAs=Null] attribute ScalarValueString treatReturnedNullStringAsNullScalarValueStringAttribute;
+    [TreatReturnedNullStringAs=Undefined] attribute ScalarValueString treatReturnedNullStringAsUndefinedScalarValueStringAttribute;
     [TypeChecking=Interface|Nullable] attribute float typeCheckingInterfaceNullableStringFloatAttribute; // nop for non-interface types
     [TypeChecking=Interface] attribute TestInterface typeCheckingInterfaceTestInterfaceAttribute;
     [TypeChecking=Nullable] attribute TestInterface? typeCheckingNullableTestInterfaceOrNullAttribute;
@@ -504,6 +506,8 @@ interface TestObject {
     [TreatReturnedNullStringAs=Undefined] DOMString treatReturnedNullStringAsUndefinedStringMethod();
     [TreatReturnedNullStringAs=Null] ByteString treatReturnedNullStringAsNullByteStringMethod();
     [TreatReturnedNullStringAs=Undefined] ByteString treatReturnedNullStringAsUndefinedByteStringMethod();
+    [TreatReturnedNullStringAs=Null] ScalarValueString treatReturnedNullStringAsNullScalarValueStringMethod();
+    [TreatReturnedNullStringAs=Undefined] ScalarValueString treatReturnedNullStringAsUndefinedScalarValueStringMethod();
     [TypeChecking=Interface] void typeCheckingInterfaceVoidMethodTestInterfaceEmptyArg(TestInterfaceEmpty testInterfaceEmptyArg);
     [TypeChecking=Nullable] void typeCheckingNullableVoidMethodTestInterfaceEmptyOrNullArg(TestInterfaceEmpty? testInterfaceEmptyOrNullArg);
     [TypeChecking=Interface|Nullable] void typeCheckingInterfaceNullableVoidMethodTestInterfaceEmptyOrNullArg(TestInterfaceEmpty? testInterfaceEmptyOrNullArg);