Add CompositeMachMessageServer and its test

util/mach/mach_message_server.h

Index: util/mach/mach_message_server.h
diff --git a/util/mach/mach_message_server.h b/util/mach/mach_message_server.h
index fb77b30c7050107a57ec62d7a354984e36d4c009..200892b28e4a932e574372e25f656beb335b19e5 100644
--- a/util/mach/mach_message_server.h
+++ b/util/mach/mach_message_server.h
@@ -17,6 +17,8 @@
 
 #include <mach/mach.h>
 
+#include <set>
+
 #include "base/basictypes.h"
 
 namespace crashpad {
@@ -68,6 +70,10 @@ class MachMessageServer {
                                            mach_msg_header_t* out,
                                            bool* destroy_complex_request) = 0;
 
+    //! \return The set of request message Mach message IDs that
+    //!     MachMessageServerFunction() is able to handle.
+    virtual std::set<mach_msg_id_t> MachMessageServerRequestIDs() = 0;
+
     //! \return The expected or maximum size, in bytes, of a request message to
     //!     be received as the \a in parameter of MachMessageServerFunction().
     virtual mach_msg_size_t MachMessageServerRequestSize() = 0;