Basic UkmService implementation

components/metrics/persisted_logs.h

Index: components/metrics/persisted_logs.h
diff --git a/components/metrics/persisted_logs.h b/components/metrics/persisted_logs.h
index 18a80008dbea37c5eda518c9584589f161a16a1e..690dee8ed5855428066e9d977793058a5cada4e2 100644
--- a/components/metrics/persisted_logs.h
+++ b/components/metrics/persisted_logs.h
@@ -76,6 +76,9 @@ class PersistedLogs {
   // Remove the staged log.
   void DiscardStagedLog();
 
+  // Delete all logs, in memory and on disk.
+  void Purge();
+
   // True if a log has been staged.
   bool has_staged_log() const { return staged_log_index_ != -1; }