Reland - Replace FakeServer's implementation with LoopbackServer invocations.

components/sync/engine_impl/loopback_server/persistent_unique_client_entity.cc

Index: components/sync/engine_impl/loopback_server/persistent_unique_client_entity.cc
diff --git a/components/sync/engine_impl/loopback_server/persistent_unique_client_entity.cc b/components/sync/engine_impl/loopback_server/persistent_unique_client_entity.cc
index 9a2b8725b17d1d4c877cc6e15c0a33f6ad4df74e..c2bdcf730d09ab3ec9ed6623ef29f6c77211185b 100644
--- a/components/sync/engine_impl/loopback_server/persistent_unique_client_entity.cc
+++ b/components/sync/engine_impl/loopback_server/persistent_unique_client_entity.cc
@@ -5,6 +5,9 @@
 #include "components/sync/engine_impl/loopback_server/persistent_unique_client_entity.h"
 
 #include "base/guid.h"
+#include "base/rand_util.h"
+#include "base/strings/string_number_conversions.h"
+#include "components/sync/base/hash_util.h"
 #include "components/sync/engine_impl/loopback_server/persistent_permanent_entity.h"
 #include "components/sync/protocol/sync.pb.h"
 
@@ -31,13 +34,21 @@ PersistentUniqueClientEntity::PersistentUniqueClientEntity(
 PersistentUniqueClientEntity::~PersistentUniqueClientEntity() {}
 
 // static
-std::unique_ptr<LoopbackServerEntity> PersistentUniqueClientEntity::Create(
+std::unique_ptr<LoopbackServerEntity>
+PersistentUniqueClientEntity::CreateFromEntity(
     const sync_pb::SyncEntity& client_entity) {
-  CHECK(client_entity.has_client_defined_unique_tag())
-      << "A PersistentUniqueClientEntity must have a client-defined unique "
-         "tag.";
   ModelType model_type = GetModelTypeFromSpecifics(client_entity.specifics());
-  string id = EffectiveIdForClientTaggedEntity(client_entity);
+  CHECK_NE(client_entity.has_client_defined_unique_tag(),
+           syncer::CommitOnlyTypes().Has(model_type))
+      << "A UniqueClientEntity should have a client-defined unique tag iff it "
+         "is not a CommitOnly type.";
+  // Without model type specific logic for each CommitOnly type, we cannot infer
+  // a reasonable tag from the specifics. We need uniqueness for how the server
+  // holds onto all objects, so simply make a new tag from a random  number.
+  string effective_tag = client_entity.has_client_defined_unique_tag()
+                             ? client_entity.client_defined_unique_tag()
+                             : base::Uint64ToString(base::RandUint64());
+  string id = LoopbackServerEntity::CreateId(model_type, effective_tag);
   return std::unique_ptr<LoopbackServerEntity>(new PersistentUniqueClientEntity(
       id, model_type, client_entity.version(), client_entity.name(),
       client_entity.client_defined_unique_tag(), client_entity.specifics(),
@@ -45,11 +56,17 @@ std::unique_ptr<LoopbackServerEntity> PersistentUniqueClientEntity::Create(
 }
 
 // static
-std::string PersistentUniqueClientEntity::EffectiveIdForClientTaggedEntity(
-    const sync_pb::SyncEntity& entity) {
-  return LoopbackServerEntity::CreateId(
-      GetModelTypeFromSpecifics(entity.specifics()),
-      entity.client_defined_unique_tag());
+std::unique_ptr<LoopbackServerEntity>
+PersistentUniqueClientEntity::CreateFromEntitySpecifics(
+    const string& name,
+    const sync_pb::EntitySpecifics& entity_specifics) {
+  ModelType model_type = GetModelTypeFromSpecifics(entity_specifics);
+  string client_defined_unique_tag = GenerateSyncableHash(model_type, name);
+  string id =
+      LoopbackServerEntity::CreateId(model_type, client_defined_unique_tag);
+  return std::unique_ptr<LoopbackServerEntity>(new PersistentUniqueClientEntity(
+      id, model_type, 0, name, client_defined_unique_tag, entity_specifics,
+      1337, 1337));
 }
 
 bool PersistentUniqueClientEntity::RequiresParentId() const {