Add an XPC implementation of sandbox::MessageServer.

sandbox/mac/xpc.h

Index: sandbox/mac/xpc.h
diff --git a/sandbox/mac/xpc.h b/sandbox/mac/xpc.h
index 1cbe9ca7d4011e5b25ccdb9d65641d079ed78bb0..5ea9f71990a0b44c0f680c39f51fca99e4ea757d 100644
--- a/sandbox/mac/xpc.h
+++ b/sandbox/mac/xpc.h
@@ -12,6 +12,8 @@
 
 #include <mach/mach.h>
 
+#include "sandbox/sandbox_export.h"
+
 // C++ library loader.
 #include "sandbox/mac/xpc_stubs.h"
 
@@ -37,4 +39,11 @@ extern "C" {
 
 #endif
 
+namespace sandbox {
+
+// Dynamically loads the XPC library.
+bool SANDBOX_EXPORT InitializeXPC();
+
+}  // namespace sandbox
+
 #endif  // SANDBOX_MAC_XPC_H_