Make attachment broker a subclass of IPC::Listener.

ipc/brokerable_attachment.cc

Index: ipc/brokerable_attachment.cc
diff --git a/ipc/brokerable_attachment.cc b/ipc/brokerable_attachment.cc
index 54c905b661e60e69fce1e4ada9988bfd8c24dd55..cf0e730a9a30620d804cd75d4f286f9b94c1afe9 100644
--- a/ipc/brokerable_attachment.cc
+++ b/ipc/brokerable_attachment.cc
@@ -21,6 +21,9 @@ BrokerableAttachment::AttachmentId GetRandomId() {
 BrokerableAttachment::BrokerableAttachment() : id_(GetRandomId()) {
 }
 
+BrokerableAttachment::BrokerableAttachment(const AttachmentId& id) : id_(id) {
+}
+
 BrokerableAttachment::~BrokerableAttachment() {
 }