[fetch] Add dialogWindow to the API (#70)

To deal with https://github.com/slightlyoff/ServiceWorker/issues/119 we need a way to set a non-worker client on a `Request` object. I suspect we want to allow these values: null, `Client` object that represents a window, `WindowProxy` object.

In a document environment it defaults to the `WindowProxy` object of global object. In a worker environment it defaults to null. When constructing a `Request` object based on another `Request` object (as e.g. `fetch()` does) we simply copy the client.

Rather than client I think we should call this new feature `dialogWindow`, since it's not equal to https://fetch.spec.whatwg.org/#concept-request-client which is used for referrer and figuring out if the fetch originated from a service worker and such. It's sole purpose is to be the target of network-spawn dialogs.

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https://github.com/whatwg/fetch/issues/70