Direct-to-cloud backup; direct-from-cloud restore

You can copy the backup file anywhere you want, e.g., to a Dropbox folder. However, it would be nice to be able to back up directly to, and restore directly from, cloud service(s). This might be Chrome (`chrome.storage.sync`) or another service.

Readers: what services would you like to backup to and restore from? Please add a comment to let me know.

My own use case requires that I not be logged in to Chrome, so `chrome.storage.sync` isn't my favorite option :) .

Some possibilities (not an exhaustive list):

- Chrome logged-in: [`chrome.storage.sync`](https://developer.chrome.com/extensions/storage#property-sync)
- Dropbox: [JS API](https://github.com/dropbox/dropbox-sdk-js)

Requested by a user via email 2017/10/25.

The issue does not name implementation files, tests, or a specific cloud service to support. First clarify the target service and backup/restore requirements, then identify the extension entry points and define completion around direct backup and restore working for the selected service.

Written by the indexing model from the issue text.