DOMException: Failed to execute 'transaction' on 'IDBDatabase': The database connection is closing.

I get this exception occasionally for the amp.dev serviceworker. I'm not 100% sure how to reproduce this. Loading the page -> clearing cache in DevTools -> reloading a few times sometimes triggers it.

```
amp-sw.js:1 Uncaught (in promise) DOMException: Failed to execute 'transaction' on 'IDBDatabase': The database connection is closing.
at https://cdn.ampproject.org/sw/amp-sw.js:1:8225
at new Promise ()
at r.transaction (https://cdn.ampproject.org/sw/amp-sw.js:1:8188)
at async r._call (https://cdn.ampproject.org/sw/amp-sw.js:1:8591)
at async r.put (https://cdn.ampproject.org/sw/amp-sw.js:1:7118)
at async i.setTimestamp (https://cdn.ampproject.org/sw/amp-sw.js:1:9212)
at async y.updateTimestamp (https://cdn.ampproject.org/sw/amp-sw.js:1:11953)
at async i.cacheDidUpdate (https://cdn.ampproject.org/sw/amp-sw.js:1:14642)
at async Object.put (https://cdn.ampproject.org/sw/amp-sw.js:1:17694)
```