IDL: Support using nullable on any method return type

Source/bindings/scripts/v8_types.py

 # Copyright (C) 2013 Google Inc. All rights reserved.
 #
 # Redistribution and use in source and binary forms, with or without
 # modification, are permitted provided that the following conditions are
 # met:
 #
 #     * Redistributions of source code must retain the above copyright
 # notice, this list of conditions and the following disclaimer.
 #     * Redistributions in binary form must reproduce the above
 # copyright notice, this list of conditions and the following disclaimer
@@ skipping 703 matching lines @@
 
 def literal_cpp_value(idl_type, idl_literal):
     """Converts an expression that is a valid C++ literal for this type."""
     # FIXME: add validation that idl_type and idl_literal are compatible
     literal_value = str(idl_literal)
     if idl_type.base_type in CPP_UNSIGNED_TYPES:
         return literal_value + 'u'
     return literal_value
 
 IdlType.literal_cpp_value = literal_cpp_value
+
+
+def is_nullable_simple(idl_type):
+    # Nullable type where the corresponding C++ type supports a null value.
+    # - String types (String/AtomicString) represent null as a null string,
+    #   i.e. one for which String::isNull() returns true.
+    # - Wrapper types (raw pointer or RefPtr/PassRefPtr) represent null as
+    #   a null pointer.
+    return idl_type.is_nullable and (
+        (idl_type.is_string_type or idl_type.is_wrapper_type) and
+        not idl_type.native_array_element_type)
+
+IdlType.is_nullable_simple = property(is_nullable_simple)
+IdlUnionType.is_nullable_simple = False