Messaging Compat check service worker environment incorrectly.

`navigator` is available in both browser and sw context, so it can't be used to determine the environment. See https://github.com/firebase/firebase-js-sdk/blob/master/packages-exp/messaging-compat/src/registerMessagingCompat.ts#L37

We can use `self && 'ServiceWorkerGlobalScope' in self` which is currently used in v8.