Reland #1: Clean up interface of attachment broker.

ipc/attachment_broker_privileged.cc

Index: ipc/attachment_broker_privileged.cc
diff --git a/ipc/attachment_broker_privileged.cc b/ipc/attachment_broker_privileged.cc
index d2b68320ba0adc9b609e927ada5dd152e1c2e67d..1fe3a75e73ec2381ea4e063bab5e5925103db465 100644
--- a/ipc/attachment_broker_privileged.cc
+++ b/ipc/attachment_broker_privileged.cc
@@ -16,6 +16,7 @@ AttachmentBrokerPrivileged::~AttachmentBrokerPrivileged() {}
 
 void AttachmentBrokerPrivileged::RegisterCommunicationChannel(
     Channel* channel) {
+  channel->set_attachment_broker_endpoint(true);
   auto it = std::find(channels_.begin(), channels_.end(), channel);
   DCHECK(channels_.end() == it);
   channels_.push_back(channel);