[w3c/ServiceWorker] The return value for update() is wrong in WebIDL (#1472)

The [current WebIDL for the `ServiceWorkerRegistration`](https://w3c.github.io/ServiceWorker/#serviceworkerregistration-interface) is incorrect; it shows `update()` returning `Promise<void>`.

<img width="857" alt="Screen Shot 2019-09-16 at 4 18 56 PM" src="https://user-images.githubusercontent.com/326742/64940018-d9f9c280-d89d-11e9-98d7-37703a8a5b79.png">

I believe this is a mistake since implementations all return the registration object, and the [`update()` algorithm](https://w3c.github.io/ServiceWorker/#dom-serviceworkerregistration-update) says to return the registration as well.




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https://github.com/w3c/ServiceWorker/issues/1472