[CrOS Tether] Add KeepAliveTickleResponse messages to both the proto file and MessageWrapper.

chromeos/components/tether/message_wrapper.h

Index: chromeos/components/tether/message_wrapper.h
diff --git a/chromeos/components/tether/message_wrapper.h b/chromeos/components/tether/message_wrapper.h
index d62ac5fb1754f46d193bc65551376c7a8339ebfe..9d92dbc1bbc75205553ed8fb8790c2d4781d9db6 100644
--- a/chromeos/components/tether/message_wrapper.h
+++ b/chromeos/components/tether/message_wrapper.h
@@ -31,6 +31,7 @@ class MessageWrapper {
   MessageWrapper(const ConnectTetheringResponse& response);
   MessageWrapper(const DisconnectTetheringRequest& request);
   MessageWrapper(const KeepAliveTickle& tickle);
+  MessageWrapper(const KeepAliveTickleResponse& response);
   MessageWrapper(const TetherAvailabilityRequest& request);
   MessageWrapper(const TetherAvailabilityResponse& response);