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

components/sync/engine_impl/loopback_server/persistent_tombstone_entity.cc

Index: components/sync/engine_impl/loopback_server/persistent_tombstone_entity.cc
diff --git a/components/sync/engine_impl/loopback_server/persistent_tombstone_entity.cc b/components/sync/engine_impl/loopback_server/persistent_tombstone_entity.cc
index a32b134c0720f4287ffc6e5407eea9c69851b6b6..f1e3c5aff66da6c03ba28e6601202eba188f0049 100644
--- a/components/sync/engine_impl/loopback_server/persistent_tombstone_entity.cc
+++ b/components/sync/engine_impl/loopback_server/persistent_tombstone_entity.cc
@@ -13,20 +13,33 @@ namespace syncer {
 PersistentTombstoneEntity::~PersistentTombstoneEntity() {}
 
 // static
-std::unique_ptr<LoopbackServerEntity> PersistentTombstoneEntity::Create(
-    const sync_pb::SyncEntity& entity) {
+std::unique_ptr<LoopbackServerEntity>
+PersistentTombstoneEntity::CreateFromEntity(const sync_pb::SyncEntity& entity) {
   const ModelType model_type = GetModelTypeFromId(entity.id_string());
   CHECK_NE(model_type, syncer::UNSPECIFIED) << "Invalid ID was given: "
                                             << entity.id_string();
   return std::unique_ptr<LoopbackServerEntity>(new PersistentTombstoneEntity(
-      entity.id_string(), entity.version(), model_type));
+      entity.id_string(), entity.version(), model_type,
+      entity.client_defined_unique_tag()));
+}
+
+// static
+std::unique_ptr<LoopbackServerEntity> PersistentTombstoneEntity::CreateNew(
+    const std::string& id,
+    const std::string& client_defined_unique_tag) {
+  const ModelType model_type = GetModelTypeFromId(id);
+  CHECK_NE(model_type, syncer::UNSPECIFIED) << "Invalid ID was given: " << id;
+  return std::unique_ptr<LoopbackServerEntity>(new PersistentTombstoneEntity(
+      id, 0, model_type, client_defined_unique_tag));
 }
 
 PersistentTombstoneEntity::PersistentTombstoneEntity(
     const string& id,
     int64_t version,
-    const ModelType& model_type)
-    : LoopbackServerEntity(id, model_type, version, string()) {
+    const ModelType& model_type,
+    const std::string& client_defined_unique_tag)
+    : LoopbackServerEntity(id, model_type, version, string()),
+      client_defined_unique_tag_(client_defined_unique_tag) {
   sync_pb::EntitySpecifics specifics;
   AddDefaultFieldValue(model_type, &specifics);
   SetSpecifics(specifics);
@@ -43,6 +56,8 @@ string PersistentTombstoneEntity::GetParentId() const {
 void PersistentTombstoneEntity::SerializeAsProto(
     sync_pb::SyncEntity* proto) const {
   LoopbackServerEntity::SerializeBaseProtoFields(proto);
+  if (!client_defined_unique_tag_.empty())
+    proto->set_client_defined_unique_tag(client_defined_unique_tag_);
 }
 
 bool PersistentTombstoneEntity::IsDeleted() const {