ipc: Make a new class PlaceholderBrokerableAttachment.

ipc/ipc.gypi

 # Copyright (c) 2012 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.
 
 {
   'target_defaults': {
     'variables': {
       'ipc_target': 0,
     },
     'target_conditions': [
@@ skipping 69 matching lines @@
           'ipc_sync_message_filter.cc',
           'ipc_sync_message_filter.h',
           'message_filter.cc',
           'message_filter.h',
           'message_filter_router.cc',
           'message_filter_router.h',
           'param_traits_log_macros.h',
           'param_traits_macros.h',
           'param_traits_read_macros.h',
           'param_traits_write_macros.h',
+          'placeholder_brokerable_attachment.cc',
+          'placeholder_brokerable_attachment.h',
           'struct_constructor_macros.h',
           'struct_destructor_macros.h',
           'unix_domain_socket_util.cc',
           'unix_domain_socket_util.h',
         ],
         'defines': [
           'IPC_IMPLEMENTATION',
         ],
         'include_dirs': [
           '..',
         ],
         'target_conditions': [
           ['>(nacl_untrusted_build)==1', {
             'sources!': [
               'ipc_channel.cc',
               'ipc_channel_posix.cc',
               'unix_domain_socket_util.cc',
             ],
           }],
           ['OS == "win" or OS == "ios"', {
             'sources!': [
               'unix_domain_socket_util.cc',
             ],
           }],
         ],
       }],
     ],
   },
 }