function enable () {
  if (powerSaveBlocker.isStarted(blockId)) {
    // If a power saver block already exists, do nothing.
    return
  }
  blockId = powerSaveBlocker.start('prevent-display-sleep')
  log(`powerSaveBlocker.enable: ${blockId}`)
}
Block the system from entering low-power (sleep) mode or turning off the
display.