Make client tag IDs consistent in sync fake server

sync/test/fake_server/fake_server.cc

Index: sync/test/fake_server/fake_server.cc
diff --git a/sync/test/fake_server/fake_server.cc b/sync/test/fake_server/fake_server.cc
index b73ef1ae7030c151dcb89a424a2a72d9f12e1a26..d5ac25d45392f25d2a2ac1f2748ead3c48d61c39 100644
--- a/sync/test/fake_server/fake_server.cc
+++ b/sync/test/fake_server/fake_server.cc
@@ -336,10 +336,11 @@ string FakeServer::CommitEntity(
         client_entity,
         entities_[client_entity.id_string()]);
   } else if (client_entity.has_client_defined_unique_tag()) {
-    if (entities_.find(client_entity.id_string()) != entities_.end()) {

[pval...(no longer on Chromium), 2014/07/23 23:54:15]

Looking back at this, perhaps I was assuming that client_entity.id_string() was
the client_defined_unique_tag itself.

[rlarocque, 2014/07/24 00:18:39]

Could be.  It's also possible that you implemented it this way because until
recently not all entities were guaranteed to have a ModelType.  Without a
ModelType, it's impossible to derive a unique client tag, so this kind of logic
would have been impossible.

+    const std::string& effective_id =
+        UniqueClientEntity::EffectiveIdForClientTaggedEntity(client_entity);
+    if (entities_.find(effective_id) != entities_.end()) {
       entity = UniqueClientEntity::CreateUpdatedVersion(
-          client_entity,
-          entities_[client_entity.id_string()]);
+          client_entity, entities_[effective_id]);
     } else {
       entity = UniqueClientEntity::CreateNew(client_entity);
     }