message: Make the deprecation warning for XLA devices appear only once
  Make "once" in "call_once" truly "once".
diff: @@ -389,7 +389,7 @@ Status XlaDevice::TryGetDeviceContext(DeviceContext** out_context) {
  // Warn about XLA_CPU/XLA_GPU exactly once.
  static void ShowXlaDeviceDeprecationWarning() {
  - absl::once_flag once;
  + static absl::once_flag once;
  absl::call_once(once, [] {
  LOG(WARNING) << "XLA_GPU and XLA_CPU devices are deprecated and will be "
  "removed in subsequent releases. Instead, use either "
  