Preserve nullability when writing idl with the ildrenderer. Use nullability to call the toNativeWit…

tools/dom/scripts/systemnative.py

Index: tools/dom/scripts/systemnative.py
diff --git a/tools/dom/scripts/systemnative.py b/tools/dom/scripts/systemnative.py
index 130c9333bd9587e1d699ae0dc635707cf8bdc454..bbd9be37b82fc11bf06ef664c1e756ed1c4d9b07 100644
--- a/tools/dom/scripts/systemnative.py
+++ b/tools/dom/scripts/systemnative.py
@@ -738,6 +738,9 @@ class DartiumBackend(HtmlDartGenerator):
         if argument.ext_attrs.get('TreatNullAs') == 'NullString':
           return True
 
+        if argument.type.nullable:
+          return True
+
         if isinstance(argument, IDLArgument):
           if IsOptional(argument) and not self._IsArgumentOptionalInWebCore(node, argument):
             return True