Slideshow block: Slides sometimes not displaying in Chrome, Safari

#### Steps to reproduce the issue

1. Set up a Slideshow block.
2. View the site in Chrome on Mac OSX, and refresh/reload a number of times.

#### What I expected
The slideshow always loads.

#### What happened instead

Very occasionally, the slideshow does not become visible. When this happens, it looks like the `data-jetpack-block-initialized` attribute gets set to true in the `wp-block-jetpack-slideshow` div, but without the `wp-swiper-initialized` or other classes being added.

This was [first reported on a Newspack site](https://therivardreport.com/), where it was happening pretty consistently. I was eventually able to trip up a jurassic ninja test site, but it took a lot of refreshes/cache clearing.

In both cases, the block displayed as expected in Firefox; the issue only seemed to happen when it did in Chrome and Safari.

Talking with @dkoo when troubleshooting, he thought it sounded like a race condition, possibly related to this line of code: https://github.com/Automattic/jetpack/blob/master/extensions/blocks/slideshow/view.js#L23.

**Screenshots**

In Chrome (v83.0.4103.97, MacOSX 10.14.6)

In Firefox (v77.0.1, Mac OSX 10.14.6)

Start with extensions/blocks/slideshow/view.js around line 23, then reproduce the issue by repeatedly refreshing the Slideshow block in Chrome or Safari. Done means the slideshow becomes visible reliably and its initialization classes, including wp-swiper-initialized, are present when initialization completes.

Written by the indexing model from the issue text.