Add CompositeMachMessageServer and its test

util/mach/child_port_server.cc

Index: util/mach/child_port_server.cc
diff --git a/util/mach/child_port_server.cc b/util/mach/child_port_server.cc
index 5f4b32242c15e72782f342f071d43020832d15b8..500cd1e57a712a859e2cee3988cd2a39940bd3a4 100644
--- a/util/mach/child_port_server.cc
+++ b/util/mach/child_port_server.cc
@@ -103,6 +103,12 @@ bool ChildPortServer::MachMessageServerFunction(
   return false;
 }
 
+std::set<mach_msg_id_t> ChildPortServer::MachMessageServerRequestIDs() {
+  const mach_msg_id_t request_ids[] = {kMachMessageIDChildPortCheckIn};
+  return std::set<mach_msg_id_t>(
+      &request_ids[0], &request_ids[arraysize(request_ids)]);
+}
+
 mach_msg_size_t ChildPortServer::MachMessageServerRequestSize() {
   return sizeof(__RequestUnion__handle_child_port_subsystem);
 }