function loadSavedState(callback) {
  chrome.storage.local.get(STATE_KEY, function (items) {
    let savedState = items[STATE_KEY];
    for (let key in StateEnum) {
      if (savedState == StateEnum[key]) {
        callback(savedState);
        return;
      }
    }
    callback(StateEnum.DISABLED);
  });
}
Loads the locally-saved state asynchronously.
@param {function} callback Callback invoked with the loaded {StateEnum}.