getSubscription() should permission check

[nikhilm]

related to #116, the spec should state to check permission and fail with PermissionDeniedError.

[martinthomson]

I think that this is simple. getSubscription doesn't need to check permissions, it only needs to retrieve the current subscription. If the origin doesn't have permission, then this retrieval had better result in null.

I think that the current steps are fine. I could add a point to the algorithm that notes that an worker without permission will always have no subscription.

[martinthomson]

I'm going to close this one. The current algorithm is correct based on my reasoning above.