The Service Worker spec has always had the (non-normative) note that "any type of synchronous requests must not be initiated inside of a service worker", to avoid blocking the service worker (as blocking the service worker would block all network requests from controlled pages). However synchronous APIs such as FileReaderSync were still available in service workers. So we'd like to fix that by removing this API from service workers. This was deprecated in Chrome 57.
Docs: https://developer.mozilla.org/en-US/docs/Web/API/FileReaderSync