[w3c/ServiceWorker] How to test "no-referrer-when-downgrade" in a web platform test (#1094)

fetch-event.https.html has a test for the referrer policy, but Chrome won't pass it because it blocks mixed content fetches by default.

The test performs a fetch to an http URL from an https iframe. It expects the service worker to receive a fetch event whose request has a `referrerPolicy` of `"no-referrer-when-downgrade"`.

However in Chrome, the fetch() will reject, with an accompanying Mixed Content warning in the console.

We could get around this by adding Chrome-specific configuration to the web platform test:
```
if (window.testRunner)
  testRunner.overridePreference('WebKitAllowRunningInsecureContent', true);
```

But this seems unfortunate. Questions:
- Does the Mixed Content spec allow Chrome to do this?
- If so, is there a better way to test behavior in the web platform test without adding Chrome-specific code?

cc/ @mkwst @yutakahirano 

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