Investigate interaction between aborted requests and the Cache API

These tests are found in [`fetch/api/abort/cache.https.any.js`](https://github.com/web-platform-tests/wpt/blob/master/fetch/api/abort/cache.https.any.js):

- Signals are not stored in the cache API
- Signals are not stored in the cache API, even if they're already aborted

It's likely our cache API implementation as a whole should be investigated to see how it stacks up against the WPT test suite.
