Issue with cacheFirst + maxAgeSeconds for CORS resource

[jeffposnick]

Library Affected:
workbox-cache-expiration

Browser & Platform:
Chrome, and others

Issue or Feature Request Description:
@pbakaus reported an issue in which a route for a CORS resource that uses cacheFirst + maxAgeSeconds isn't behaving like it should. The configuration is https://github.com/ampproject/amp-publisher-sample/blob/master/amp-pwa-reader/src/sw.js#L18, and a live example is at https://amp.cards/

From what I can tell, the original response is reused indefinitely, instead of expiring after 5 minutes have passed.