constructor(opt_callback) {
    super();
    this.storage_ = {};
    this.updateStorage_(opt_callback);
    chrome.storage.onChanged.addListener(goog.bind(function(changes, areaName) {
      var keyName = e2e.ext.util.ChromeStorageLocal.STORAGE_KEY_NAME_;
      if (areaName == 'local' && keyName in changes) {
        this.storage_ = changes[keyName].newValue || {};
      }
    }, this));
  }
@param {function(!e2e.ext.util.ChromeStorageLocal)=} opt_callback Callback
    function to call once storage mechanism has been initialized.