Pointed out in a comment on https://crrev.com/c/1188472 (after landing):
That CL introduced a situation where SyncAuthManager drops its access token (because it's invalid/expired), but does not tell IdentityManager (-> OAuth2TokenService) about it. That means the next token request we send will likely give us back the very same token - which we'd again drop, rinse and repeat.

I haven't heard of any practical problems caused by this: Generally O2TS should notice itself that a token has expired and drop it automatically. Still, better to be safe (and consistent).

The short-term fix is simply to tell IdentityManager to drop the token too. Longer-term, maybe SyncAuthManager doesn't need to cache the token at all, since the identity system caches it anyway.