message: Don't consume the Undo key event when it's no-op
  This way, the client can handle the unconsumed Undo event (e.g., send it back to the app).
diff: @@ -2219,7 +2219,10 @@ bool Session::UndoOrRewind(commands::Command *command) {
  return Undo(command);
  }
  - return DoNothing(command);
  + // Mozc decoder doesn't do anything for UNDO_OR_REWIND.
  + // Echo back the event to the client to give it a chance to delegate
  + // undo operation to the app.
  + return EchoBack(command);
  }
  bool Session::SendComposerCommand(
  