Basic UkmService implementation

components/metrics/persisted_logs.cc

Index: components/metrics/persisted_logs.cc
diff --git a/components/metrics/persisted_logs.cc b/components/metrics/persisted_logs.cc
index a407a0d46970e04f192011fd047bf7545d84c668..922a296f94742494bb0837e7e291933857530eed 100644
--- a/components/metrics/persisted_logs.cc
+++ b/components/metrics/persisted_logs.cc
@@ -96,14 +96,14 @@ void PersistedLogs::SerializeLogs() const {
 
   // After writing all the logs to the new pref remove old outdated pref.
   // TODO(gayane): Remove when all users are migrated. crbug.com/649440
-  if (local_state_->HasPrefPath(outdated_pref_name_))
+  if (outdated_pref_name_ && local_state_->HasPrefPath(outdated_pref_name_))
     local_state_->ClearPref(outdated_pref_name_);
 }
 
 PersistedLogs::LogReadStatus PersistedLogs::DeserializeLogs() {
   // TODO(gayane): Remove the code for reading logs from outdated pref when all
   // users are migrated. crbug.com/649440
-  if (local_state_->HasPrefPath(outdated_pref_name_)) {
+  if (outdated_pref_name_ && local_state_->HasPrefPath(outdated_pref_name_)) {
     return ReadLogsFromOldFormatPrefList(
         *local_state_->GetList(outdated_pref_name_));
   }