Masterbar: logging out of your site does not log you out of WordPress.com

[jeherve]

Steps to reproduce the issue

1. Check out Masterbar: Revert recent changes #12454
2. Activate the WordPress.com Toolbar feature on your site, under Jetpack > Settings > Writing
3. Make sure you are logged in with your WordPress.com account.
4. In the masterbar, click on your gravatar.
5. Click sign out.
6. You will be signed out of your site, but not from WordPress.com.

In fact, as soon as you log out the Masterbar module stops loading, and thus this is never triggered:

add_action( 'wp_logout', array( $this, 'maybe_logout_user_from_wpcom' ) );

-- https://github.com/Automattic/jetpack/pull/12454/files#diff-d73a71fc4f0617de9989f4321bc4f39cR145

This appears to be caused by #10945 and #11088, which defered loading of the module altogether. Logging out of WordPress.com worked before those changes.