Re-land: bindings: Use MessageEventInit for MessageEvent constructor

Source/bindings/scripts/v8_dictionary.py

 # Copyright 2014 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 """Generate template contexts of dictionaries for both v8 bindings and
 implementation classes that are used by blink's core/modules.
 """
 
 import operator
 from idl_types import IdlType
@@ skipping 179 matching lines @@
     if member.default_value and not member.default_value.is_null:
         cpp_default_value = idl_type.literal_cpp_value(member.default_value)
 
     header_includes.update(idl_type.impl_includes_for_type(interfaces_info))
     return {
         'cpp_default_value': cpp_default_value,
         'cpp_name': cpp_name,
         'getter_expression': getter_expression(),
         'has_method_expression': has_method_expression(),
         'has_method_name': has_method_name_for_dictionary_member(member),
+        'is_nullable': idl_type.is_nullable,
         'is_traceable': idl_type.is_traceable,
         'member_cpp_type': member_cpp_type(),
         'null_setter_name': null_setter_name_for_dictionary_member(member),
         'rvalue_cpp_type': idl_type.cpp_type_args(used_as_rvalue_type=True),
         'setter_name': setter_name_for_dictionary_member(member),
     }