Javascript deprecation notice: Synchronous XMLHttpRequest on the main thread

#### Steps to reproduce the issue

1. Go to https://www.deeblogi.fi with Chrome 78 (beta) or Firefox 69
2. Open browser console
3. See Javascript notice

#### What I expected
No Javascript notices

#### What happened instead
There's a following notice message in console:
```
??/wp-content/js/postmessage.js,/wp-content/js/jed/jed.js,/wp-content/js/wpcom-proxy-request.js,/wp-content/js/likes-rest-nojquery.js?m=20190321:2999

[Deprecation] Synchronous XMLHttpRequest on the main thread is deprecated because of its detrimental effects to the end user's experience. For more help, check https://xhr.spec.whatwg.org/.

wpLikes.displayWidget @ ??/wp-content/js/postmessage.js,/wp-content/js/jed/jed.js,/wp-content/js/wpcom-proxy-request.js,/wp-content/js/likes-rest-nojquery.js?m=20190321:2999

ajax @ ??/wp-content/js/postmessage.js,/wp-content/js/jed/jed.js,/wp-content/js/wpcom-proxy-request.js,/wp-content/js/likes-rest-nojquery.js?m=20190321:2206

getPostLikes @ ??/wp-content/js/postmessage.js,/wp-content/js/jed/jed.js,/wp-content/js/wpcom-proxy-request.js,/wp-content/js/likes-rest-nojquery.js?m=20190321:2122

readMessage @ ??/wp-content/js/postmessage.js,/wp-content/js/jed/jed.js,/wp-content/js/wpcom-proxy-request.js,/wp-content/js/likes-rest-nojquery.js?m=20190321:2468

(anonymous) @ ??/wp-content/js/postmessage.js,/wp-content/js/jed/jed.js,/wp-content/js/wpcom-proxy-request.js,/wp-content/js/likes-rest-nojquery.js?m=20190321:3304

_dispatch @ ??/wp-content/js/postmessage.js,/wp-content/js/jed/jed.js,/wp-content/js/wpcom-proxy-request.js,/wp-content/js/likes-rest-nojquery.js?m=20190321:251
```

#### WordPress and plugin version
WordPress: 5.2.3
Jetpack: 7.7.2

Start by tracing the console stack through postmessage.js, jed.js, wpcom-proxy-request.js, and likes-rest-nojquery.js, focusing on wpLikes.displayWidget, ajax, and getPostLikes. Reproduce the notice in Chrome or Firefox and identify the synchronous XMLHttpRequest path. Done means the reproduction no longer emits this deprecation notice.

Written by the indexing model from the issue text.