The MetricsLogStore handles all serialized logs, while MetricsLogManager
handles logs which are still being produced.

Creates a LogStore interface for MetricsLogStore and PersistedLogs to
implement.  This will simplify creating a ReportingService that can be reused
by both UKM and UMA.

No functional changes intended.

BUG=693676

Review-Url: https://codereview.chromium.org/2689323010
Cr-Commit-Position: refs/heads/master@{#452356}