Add CompositeMachMessageServer and its test

util/mach/exc_server_variants.h

Index: util/mach/exc_server_variants.h
diff --git a/util/mach/exc_server_variants.h b/util/mach/exc_server_variants.h
index 80fb659c080f7264b6ae0c3324a1d37556950812..3552f99f26b67ca2196e0dfe922da6d3c02be2b1 100644
--- a/util/mach/exc_server_variants.h
+++ b/util/mach/exc_server_variants.h
@@ -17,6 +17,8 @@
 
 #include <mach/mach.h>
 
+#include <set>
+
 #include "build/build_config.h"
 #include "util/mach/mach_message_server.h"
 
@@ -118,6 +120,8 @@ class ExcServer : public MachMessageServer::Interface {
                                  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;
 
@@ -217,6 +221,8 @@ class MachExcServer : public MachMessageServer::Interface {
                                  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;
 
@@ -483,6 +489,8 @@ class UniversalMachExcServer
                                  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;