[ServiceWorker] IDL: define ContextFrameType (#572)

The frameType attribute of [ServiceWorkerClient](https://slightlyoff.github.io/ServiceWorker/spec/service_worker/#service-worker-client-interface) is declared as [ContextFrameType](http://fetch.spec.whatwg.org/#concept-request-context-frame-type), which is not a part of the interface of the fetch spec but is an internal concept.

We should have a definition of ContextFrameType enum somewhere.


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https://github.com/slightlyoff/ServiceWorker/issues/572