[Sync] Reimplement proto value conversions on top of field visitors.

components/sync/engine/cycle/sync_cycle_snapshot.cc

Index: components/sync/engine/cycle/sync_cycle_snapshot.cc
diff --git a/components/sync/engine/cycle/sync_cycle_snapshot.cc b/components/sync/engine/cycle/sync_cycle_snapshot.cc
index 27cffce49b4e985e3e4ddc70c3c62806028cf07e..765e120e2f07227da2625bfb3e77b1bb6de527f5 100644
--- a/components/sync/engine/cycle/sync_cycle_snapshot.cc
+++ b/components/sync/engine/cycle/sync_cycle_snapshot.cc
@@ -82,8 +82,7 @@ std::unique_ptr<base::DictionaryValue> SyncCycleSnapshot::ToValue() const {
   value->SetInteger("numHierarchyConflicts", num_hierarchy_conflicts_);
   value->SetInteger("numServerConflicts", num_server_conflicts_);
   value->SetInteger("numEntries", num_entries_);
-  value->SetString("legacySource",
-                   GetUpdatesSourceString(legacy_updates_source_));
+  value->SetString("legacySource", ProtoEnumToString(legacy_updates_source_));
   value->SetBoolean("notificationsEnabled", notifications_enabled_);
 
   std::unique_ptr<base::DictionaryValue> counter_entries(