IPC: Add new classes required for attachment brokering.

ipc/brokerable_attachment.cc

Index: ipc/brokerable_attachment.cc
diff --git a/ipc/brokerable_attachment.cc b/ipc/brokerable_attachment.cc
new file mode 100644
index 0000000000000000000000000000000000000000..be5d51b2069451e25eb1ff987fb48bfd32e9b166
--- /dev/null
+++ b/ipc/brokerable_attachment.cc
@@ -0,0 +1,19 @@
+// Copyright 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/brokerable_attachment.h"
+
+namespace IPC {
+
+BrokerableAttachment::BrokerableAttachment() {
+}
+
+BrokerableAttachment::~BrokerableAttachment() {
+}
+
+BrokerableAttachment::AttachmentId BrokerableAttachment::GetIdentifier() const {
+  return id_;
+}
+
+}  // namespace IPC