[mojo-edk] Add support for transferring mach ports.

mojo/edk/system/BUILD.gn

Index: mojo/edk/system/BUILD.gn
diff --git a/mojo/edk/system/BUILD.gn b/mojo/edk/system/BUILD.gn
index b9cac173358a96d63140c1ec8c301cbbe72d33c4..b2d416f161d255983e79940e7e262405474e27a1 100644
--- a/mojo/edk/system/BUILD.gn
+++ b/mojo/edk/system/BUILD.gn
@@ -105,6 +105,13 @@ component("system") {
                             # which is uninteresting.
   }
 
+  if (is_mac && !is_ios) {
+    sources += [
+      "mach_port_relay.cc",
+      "mach_port_relay.h",
+    ]
+  }
+
   allow_circular_includes_from = [ "//mojo/edk/embedder" ]
 }