[bindings] Use |PostMessage| attribute for Client.postMessage to eliminate custom bindings

Source/bindings/modules/v8/custom/custom.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.
 
 {
     'variables': {
         # Note: list duplicated in custom.gni.
         'bindings_modules_v8_custom_files': [
-            'V8ClientCustom.cpp',
             'V8CustomSQLStatementErrorCallback.cpp',
             'V8DeviceMotionEventCustom.cpp',
             'V8DeviceOrientationEventCustom.cpp',
         ],
     },
 }