[bindings] Support is_explicit_nullable for method arguments.

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 916 matching lines @@
 def cpp_type_has_null_value(idl_type):
     # - String types (String/AtomicString) represent null as a null string,
     #   i.e. one for which String::isNull() returns true.
     # - Enum types, as they are implemented as Strings.
     # - Interface types (raw pointer or RefPtr/PassRefPtr) represent null as
     #   a null pointer.
     # - Union types, as thier container classes can represent null value.
     # - 'Object' type. We use ScriptValue for object type.
     return (idl_type.is_string_type or idl_type.is_interface_type or
             idl_type.is_enum or idl_type.is_union_type
-            or idl_type.base_type == 'object')
+            or idl_type.base_type == 'object'
+            or idl_type.is_callback_function or idl_type.is_callback_interface)
 
 IdlTypeBase.cpp_type_has_null_value = property(cpp_type_has_null_value)
 
 
 def is_implicit_nullable(idl_type):
     # Nullable type where the corresponding C++ type supports a null value.
     return idl_type.is_nullable and idl_type.cpp_type_has_null_value
 
 
 def is_explicit_nullable(idl_type):
@@ skipping 21 matching lines @@
     number_of_nullable_member_types_union)
 
 
 def includes_nullable_type_union(idl_type):
     # http://heycam.github.io/webidl/#dfn-includes-a-nullable-type
     return idl_type.number_of_nullable_member_types == 1
 
 IdlTypeBase.includes_nullable_type = False
 IdlNullableType.includes_nullable_type = True
 IdlUnionType.includes_nullable_type = property(includes_nullable_type_union)