FetchEvent: add origin check before setting replacesClientId

[gterzian]

As part of implementing replacesClientId, the issue of what to do in the event of a cross-origin navigation came up. In such a case, replacesClientId will be the id of a client whose origin is different from the service worker handling the fetch event.

So, it feels as if Step 24.3.9 of Handle Fetch should include a same origin check and initialize replacesClientId to the empty string if it fails?

I can also write additional tests for this specific situation as part of WebKit/WebKit#13130

[gterzian]

cc @youennf