message: Don't copy the target_device string every time it is accessed.
diff: @@ -376,7 +376,7 @@ class ProcessFunctionLibraryRuntime {
  local_handle_(local_handle),
  function_key_(function_key) {}
  - string target_device() { return target_device_; }
  + const string& target_device() { return target_device_; }
  const string& function_key() { return function_key_; }
  FunctionLibraryRuntime::LocalHandle local_handle() {
  