Add some v8 bindings files to GN build.

Source/bindings/modules/v8/v8.gypi

 # 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.
 
 {
     'includes': [
         'custom/custom.gypi',
     ],
     'variables': {
+        # Note: list duplicated in v8.gni.
         'bindings_modules_v8_files': [
             '<@(bindings_modules_v8_custom_files)',
             'DictionaryHelperForModules.cpp',
             'IDBBindingUtilities.cpp',
             'IDBBindingUtilities.h',
             'ModuleBindingsInitializer.cpp',
             'ModuleBindingsInitializer.h',
         ],
+        # Note: list duplicated in v8.gni.
         'bindings_modules_v8_unittest_files': [
             'IDBBindingUtilitiesTest.cpp',
         ],
     },
 }