VideoPress + Classic block = no preview on Edit page

## Summary
VideoPress fails to show a preview in the "Edit" page when using the Classic block.

## Steps to Reproduce

1. Add a Classic block.
2. Insert media/upload video.

### Expected Result
1. Displays a shortcode and/or a preview

### Actual Result
1. Initially displays a shortcode when still on the Editor page

Image Link: http://cld.wthms.co/zEdzis

2. Upon reload/revisit of this page, it is blank instead

Image Link: http://cld.wthms.co/zrs9VC

## Site Info

- URL: https://test.support/2019/01/02/videopress-classic-block-in-gutenberg/
- JP Debug: https://jetpack.com/support/debug/?url=https%3A%2F%2Ftest.support%2F
- User: `mzakariya`
- Ticket/Forum/Chat: 1672306-zen

Reproduce the problem by adding a Classic block, inserting or uploading a video, and revisiting the Edit page. The issue names no source file or test entry point; done means the VideoPress content shows a shortcode and/or preview instead of becoming blank after reload.

Written by the indexing model from the issue text.