[ServiceWorker] What is FetchEvent.clientId for navigation requests (#808)

Spin-off of issue #723 

The spec recently made clientId == null for navigation requests:
https://github.com/slightlyoff/ServiceWorker/commit/872e51b9f06d9872d925a2877f02ca765194ef78

But if I'm not mistaken, Gecko populates clientId[1] on the navigation request and re-uses it for subresource requests from that client:
```
79       .then(function() { return with_iframe(scope); })
80       .then(function(f) {
81           frame = f;
82           assert_equals(
83             frame.contentDocument.body.textContent.substr(0, 15),
84             'Client ID Found',
85             'Service Worker should respond to fetch with a client id');
86           initial_client_id = frame.contentDocument.body.textContent.substr(17);
```
[1] http://lxr.mozilla.org/mozilla-central/source/testing/web-platform/mozilla/tests/service-workers/service-worker/fetch-event.https.html?force=1

What is the desired behavior? cc @jungkees @wanderview 



---
Reply to this email directly or view it on GitHub:
https://github.com/slightlyoff/ServiceWorker/issues/808