ipc: Make a new class PlaceholderBrokerableAttachment.

ipc/BUILD.gn

 # 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.
 
 import("//testing/test.gni")
 
 component("ipc") {
   sources = [
     "attachment_broker.cc",
     "attachment_broker.h",
@@ skipping 64 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",
   ]
 
   if (is_nacl) {
     sources -= [
       "ipc_channel.cc",
       "ipc_channel_posix.cc",
@@ skipping 121 matching lines @@
     "ipc_test_sink.cc",
     "ipc_test_sink.h",
   ]
   deps = [
     ":ipc",
     "//base",
     "//base/test:test_support",
     "//testing/gtest",
   ]
 }