Publicize: wpas_default_prefix, wpas_default_message, and wpas_default_suffix not working in Gutenberg

#### Steps to reproduce the issue

1. Install latest WP
2. Install Jetpack
3. Activate Publicize module
4. Connect Twitter account to Jetpack
5. Install Gutenberg plugin
6. Install a functional plugin (Code Snippets)
7. Create a new snippet with the following code in Code Snippets:
```php
function set_new_publicize_default_prefix($msg) {
return 'prefix-';
}
add_filter('wpas_default_prefix', 'set_new_publicize_default_prefix');

function set_new_publicize_default_msg($msg) {
return 'default';
}
add_filter('wpas_default_message', 'set_new_publicize_default_msg');

function set_new_publicize_default_suffix($msg) {
return '-suffix';
}
add_filter('wpas_default_suffix', 'set_new_publicize_default_suffix');
```
8. Save & Activate code snippet
9. Create new post with Gutenberg (use any title)
10. Attempt to publish post

#### What I expected
I expected to see, in the "Customize your Message" text area, under "Share This Post" in the "Publish Settings" dialog of Gutenberg, the text `prefix-default-suffix`.

#### What happened instead
What I see in the "Customize your Message" area is the title of the post, instead.

**Screenshots**

Reproduce the issue with Jetpack Publicize, a connected Twitter account, and Gutenberg using the listed PHP filters. Trace how the Publicize defaults reach Gutenberg's Publish Settings dialog; done means the Customize your Message area shows `prefix-default-suffix` instead of the post title.

Written by the indexing model from the issue text.