[sync] Reland - Replace FakeServer's implementation with LoopbackServer invocations.

components/sync/engine_impl/loopback_server/persistent_unique_client_entity.h

Index: components/sync/engine_impl/loopback_server/persistent_unique_client_entity.h
diff --git a/components/sync/engine_impl/loopback_server/persistent_unique_client_entity.h b/components/sync/engine_impl/loopback_server/persistent_unique_client_entity.h
index 8e039002b68d3ab50c04d023bf207753105c5394..19cb1bf732f9887c4c2a1add110a8a483bf44422 100644
--- a/components/sync/engine_impl/loopback_server/persistent_unique_client_entity.h
+++ b/components/sync/engine_impl/loopback_server/persistent_unique_client_entity.h
@@ -39,9 +39,15 @@ class PersistentUniqueClientEntity : public LoopbackServerEntity {
   ~PersistentUniqueClientEntity() override;
 
   // Factory function for creating a PersistentUniqueClientEntity.
-  static std::unique_ptr<LoopbackServerEntity> Create(
+  static std::unique_ptr<LoopbackServerEntity> CreateFromEntity(
       const sync_pb::SyncEntity& client_entity);
 
+  // Factory function for creating a PersistentUniqueClientEntity for use in the
+  // FakeServer injection API.
+  static std::unique_ptr<LoopbackServerEntity> CreateFromEntitySpecifics(
+      const std::string& name,
+      const sync_pb::EntitySpecifics& entity_specifics);
+
   // Derives an ID from a unique client tagged entity.
   static std::string EffectiveIdForClientTaggedEntity(
       const sync_pb::SyncEntity& entity);