[w3c/ServiceWorker] activation of the Service Worker on an iframe context (#1029)

### Before Chome 54:

- register to a Service Worker with a `domain.com` scope
- go to an HTTPS page with a different domain (`other-domain.com`), that include an HTTPS iframe (`domain.com`) that fall under the Service Worker scope
- the `domain.com` Service Worker is activated and is **running**

### Since Chrome 54:

- register to a Service Worker with a `domain.com` scope
- go to an HTTPS page with a different domain (`other-domain.com`), that include an HTTPS iframe (`domain.com`) that fall under the Service Worker scope
- the `domain.com` Service Worker is activated and is **stopped**
- Service Worker events **will force it to run** (e.g. the installation of a new version, or even a `postMessage()` from the iframe)

### Other contexts
- with Firefox (tested on the FirefoxDeveloperEdition 52), the Service Worker will be activated and **running**
- same behavior if we visit the `domain.com` directly, i.e. without the iframe context, the Service Worker is **running** (we can take this [example](https://jakearchibald.github.io/trained-to-thrill/) from Jake Archibald)


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