async setBackgroundSync(flag) {
        return await this.pupPage.evaluate(async flag => {
            const backSync = window.Store.Settings.getGlobalOfflineNotifications();
            if (backSync === flag) {
                return flag;
            }
            await window.Store.Settings.setGlobalOfflineNotifications(flag);
            return flag;
        }, flag);
    }
Setting background synchronization.
NOTE: this action will take effect after you restart the client.
@param {boolean} flag true/false
@returns {Promise<boolean>}