Reland #1: Clean up interface of attachment broker.

ipc/attachment_broker_unprivileged.cc

Index: ipc/attachment_broker_unprivileged.cc
diff --git a/ipc/attachment_broker_unprivileged.cc b/ipc/attachment_broker_unprivileged.cc
index bc2eb5a764dbf0dfafa61ea5286bbc04d1b4bb70..f12cfe2748cd62a26666b58af804d925c6060d85 100644
--- a/ipc/attachment_broker_unprivileged.cc
+++ b/ipc/attachment_broker_unprivileged.cc
@@ -4,10 +4,21 @@
 
 #include "ipc/attachment_broker_unprivileged.h"
 
+#include "ipc/ipc_channel.h"
+
 namespace IPC {
 
-AttachmentBrokerUnprivileged::AttachmentBrokerUnprivileged() {}
+AttachmentBrokerUnprivileged::AttachmentBrokerUnprivileged()
+    : sender_(nullptr) {}
 
 AttachmentBrokerUnprivileged::~AttachmentBrokerUnprivileged() {}
 
+void AttachmentBrokerUnprivileged::DesignateBrokerCommunicationChannel(
+    IPC::Channel* channel) {
+  DCHECK(channel);
+  DCHECK(!sender_);
+  sender_ = channel;
+  channel->set_attachment_broker_endpoint(true);
+}
+
 }  // namespace IPC