- Use FakeDataSource as the upstream source.
- Actually validate that caching is happening (i.e. reads happen
on the upstream source only if the data hasn't been read through
the CacheDataSource already).
- Move FakeClock to sit alongside the other Fake classes.
-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=118555903
