Re: [ServiceWorker] add [NewObject] where appropriate (#692)

@wanderview Thanks for the suggestion. One thing to check before adding this is this rule specified in WebIDL: "The [NewObject] extended attribute must not be used on anything other than a regular or static operation **whose return type is an interface type**." (http://heycam.github.io/webidl/#NewObject)

I found @annevk used it for Promise type in fetch spec though. (https://fetch.spec.whatwg.org/#globalfetch)

/cc @annevk @heycam

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