JS binding: remove redundant type check in attr setter for TypeChecking interface

Source/bindings/scripts/v8_attributes.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 305 matching lines @@
     # [RaisesException], [RaisesException=Setter]
     is_setter_raises_exception = (
         'RaisesException' in extended_attributes and
         extended_attributes['RaisesException'] in [None, 'Setter'])
     # [TypeChecking=Interface]
     has_type_checking_interface = (
         (has_extended_attribute_value(interface, 'TypeChecking', 'Interface') or
          has_extended_attribute_value(attribute, 'TypeChecking', 'Interface')) and
         idl_type.is_wrapper_type)
 
+    type_checked = (has_type_checking_interface and
+                    # These allow null values, so a type-check is still required.
+                    not idl_type.is_nullable)
+
     context.update({
         'has_setter_exception_state':
             is_setter_raises_exception or has_type_checking_interface or
             context['has_type_checking_unrestricted'] or
             idl_type.v8_conversion_needs_exception_state,
         'has_type_checking_interface': has_type_checking_interface,
         'is_setter_call_with_execution_context': v8_utilities.has_extended_attribute_value(
             attribute, 'SetterCallWith', 'ExecutionContext'),
         'is_setter_raises_exception': is_setter_raises_exception,
         'private_script_cpp_value_to_v8_value': idl_type.cpp_value_to_v8_value(
             'cppValue', isolate='scriptState->isolate()',
             creation_context='scriptState->context()->Global()'),
         'v8_value_to_local_cpp_value': idl_type.v8_value_to_local_cpp_value(
-            extended_attributes, 'v8Value', 'cppValue'),
+            extended_attributes, 'v8Value', 'cppValue',
+            needs_type_check=not type_checked),
     })
 
     # setter_expression() depends on context values we set above.
     context['cpp_setter'] = setter_expression(interface, attribute, context)
 
 
 def setter_expression(interface, attribute, context):
     extended_attributes = attribute.extended_attributes
     arguments = v8_utilities.call_with_arguments(
         extended_attributes.get('SetterCallWith') or
@@ skipping 143 matching lines @@
     lambda self: strip_suffix(self.base_type, 'Constructor'))
 
 
 def is_constructor_attribute(attribute):
     # FIXME: replace this with [ConstructorAttribute] extended attribute
     return attribute.idl_type.name.endswith('Constructor')
 
 
 def constructor_getter_context(interface, attribute, context):
     context['needs_constructor_getter_callback'] = context['measure_as'] or context['deprecate_as']