Don't require Cache API to have a secure context

[aerik]

Posting new issue as WICG/background-fetch#165 was closed with the only comment being that it was in the wrong section

It seems to only require a secure context due to being bundled with Service Workers, but Service Workers are not required for the cache API to be used or have utility. A note on the MDN page (https://developer.mozilla.org/en-US/docs/Web/API/CacheStorage) cryptically suggests that permissions for the Cache API may be "more complex in the future".

There are a number of issues discussing the impact gating features to a secure context either crippling functionality, creating an undue burden (or impossible where no domain name is used) of assigned certificates, or of encouraging users to ignore the warnings when a self signed cert is used. One example is here: w3c/webappsec-secure-contexts#60

Splitting the cache API into it's own spec (#879) would remove the conceptual bundling and reflect the fact that the cache API doesn't need Service Workers.

Further, indexedDB does not require a secure context and has it's own spec (https://w3c.github.io/IndexedDB/) so treating Cache API similarly makes sense.