Otherwise, for resources like images loaded via <img> tags, the browser
sets `mode: "no-cors"` (as these aren't CORS requests), so the service
worker gets an opaque response even though the server sends CORS
headers.
We could upgrade all no-cors requests to cors mode, sending a `mode:
"cors"` request to a server that doesn't send CORS headers will fail
entirely: the `fetch` call throws a `TypeError` (network error), and the
browser blocks the response. So the resource wouldn't load at all, not
even as opaque. We wouldn't be able to cache it at all.
By opting in via `fetchOptions`, we can do it only for rules where we
know the server will send CORS headers.
Rosa Gutierrez
67d629f722