IPC: Introduce MessageAttachment and PlatformFileAttachment.

ipc/ipc_message_attachment.cc

Index: ipc/ipc_message_attachment.cc
diff --git a/ipc/ipc_message_attachment.cc b/ipc/ipc_message_attachment.cc
new file mode 100644
index 0000000000000000000000000000000000000000..83440ae8e008847eebd411f8ffd08c69d9f529fe
--- /dev/null
+++ b/ipc/ipc_message_attachment.cc
@@ -0,0 +1,15 @@
+// Copyright (c) 2015 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.
+
+#include "ipc/ipc_message_attachment.h"
+
+namespace IPC {
+
+MessageAttachment::MessageAttachment() {
+}
+
+MessageAttachment::~MessageAttachment() {
+}
+
+}  // namespace IPC