It would be good to clean up our code base under src/platform2/diagnostics, now that new tools from libchrome became available.

Some examples that come to mind:
1. base::Callback -> OnceCallback|RepeatingCallback.
2. base::LazyInstance -> base::NoDestructor.
3. std::atomic_flag -> base::AtomicFlag.
4. std::unique_ptr -> base::Optional (* where the semantics makes sense).
5. Get rid of brillo/bind_lambda.h usages (however, maybe only possible for captureless lambdas).
6. base::MessageLoop::current -> base::ThreadTaskRunnerHandle::Get.