Make Dispatcher::Type an enum class.

mojo/edk/system/message_in_transit.cc

Index: mojo/edk/system/message_in_transit.cc
diff --git a/mojo/edk/system/message_in_transit.cc b/mojo/edk/system/message_in_transit.cc
index 47148f34b38289d9a1894b582a6f01a6217a8adf..490933624d1679a42f5e5a61c0e9c22d973e4c91 100644
--- a/mojo/edk/system/message_in_transit.cc
+++ b/mojo/edk/system/message_in_transit.cc
@@ -205,13 +205,5 @@ void MessageInTransit::UpdateTotalSize() {
   }
 }
 
-std::ostream& operator<<(std::ostream& out, MessageInTransit::Type type) {
-  return out << static_cast<uint16_t>(type);
-}
-
-std::ostream& operator<<(std::ostream& out, MessageInTransit::Subtype subtype) {
-  return out << static_cast<uint16_t>(subtype);
-}
-
 }  // namespace system
 }  // namespace mojo