Fix the TODO:
    // TODO(falken): This and below should probably be NotifyStartError, not
    // DeliverErrorResponse. But changing it causes
    // ServiceWorkerURLRequestJobTest.DeletedProviderHostBeforeFetchEvent to
    // fail.

This is a weird way to return a network error. We should be doing NotifyStartError instead.

Currently we're weirdly adding ServiceWorkerResponseInfo saying the request was intercepted by SW , etc, etc, to this 500 response in OnStartCompleted() when a lot of the values (like WorkerPreparationType) will be invalid.