Re-enable firestore persistence with enableIndexedDbPersistence

[jkomoros]

Persistence was turned off in #528 , because enableIndexedDbPersistence 's promise was never being resolved or rejected when running in a browser where there had been persistence enabled with the v8 firestore.

It turns out to not be that big of a deal; even without persistence the cache updates happen as soon as the batch is committed, before the server fully acks, so the UI can update quickly.

Presumably this might make reloads of the app slower, but in practice it didn't seem like it make a big difference in the first place. Tracking just for completeness.