Add CompositeMachMessageServer and its test

util/mach/child_port_server_test.cc

Index: util/mach/child_port_server_test.cc
diff --git a/util/mach/child_port_server_test.cc b/util/mach/child_port_server_test.cc
index ddc525bad9fd8929246996dca3c11b5d994e2450..b9ade37745e99e0fda489bf945f9348c90f5ab74 100644
--- a/util/mach/child_port_server_test.cc
+++ b/util/mach/child_port_server_test.cc
@@ -101,6 +101,10 @@ TEST(ChildPortServer, MockChildPortCheckIn) {
   MockChildPortServerInterface server_interface;
   ChildPortServer server(&server_interface);
 
+  std::set<mach_msg_id_t> expect_request_ids;
+  expect_request_ids.insert(10011);  // There is no constant for this.
+  EXPECT_EQ(expect_request_ids, server.MachMessageServerRequestIDs());
+
   ChildPortCheckInRequest request;
   EXPECT_LE(request.Head.msgh_size, server.MachMessageServerRequestSize());