AwMetricsServiceClient::LoadOrCreateClientId() loads the client ID for metrics.

Variations also uses the client ID, to seed its EntropyProvider. Since adding variations, we also "pre-load" the client ID in AwMetricsServiceClient.preloadClientId. This happens before native initialization. If successful, LoadOrCreateClientId uses the pre-loaded value.

We're keeping the loading code in both places for now, but if we decide we're happy with the variations implementation, we should unify them.

This will require either porting base::IsValidGUID to Java or moving the load to somewhere in native before the client ID is needed by AwFieldTrialCreator.