Add NotifyServer and its test

util/mach/child_port_server.h

Index: util/mach/child_port_server.h
diff --git a/util/mach/child_port_server.h b/util/mach/child_port_server.h
index 7c40809c88bf120611a4d6f743d01373355f4b87..bc42cc094f56614e6eacace914f7da2aa335c458 100644
--- a/util/mach/child_port_server.h
+++ b/util/mach/child_port_server.h
@@ -59,13 +59,10 @@ class ChildPortServer : public MachMessageServer::Interface {
   explicit ChildPortServer(Interface* interface);
 
   // MachMessageServer::Interface:
-
   bool MachMessageServerFunction(const mach_msg_header_t* in_header,
                                  mach_msg_header_t* out_header,
                                  bool* destroy_complex_request) override;
-
   std::set<mach_msg_id_t> MachMessageServerRequestIDs() override;
-
   mach_msg_size_t MachMessageServerRequestSize() override;
   mach_msg_size_t MachMessageServerReplySize() override;