[Sync] USS: Add SMTP tests, clean up constants, etc.

BUG=569636,569678

Review URL: https://codereview.chromium.org/1800313002

Cr-Commit-Position: refs/heads/master@{#381518}

sync/internal_api/shared_model_type_processor.cc

@@ -393,8 +393,7 @@ void SharedModelTypeProcessor::OnUpdateReceived(
       DVLOG(2) << ModelTypeToString(type_) << ": Requesting re-encrypt commit "
                << update.encryption_key_name << " -> "
                << data_type_state_.encryption_key_name();
-      auto it2 = entities_.find(client_tag_hash);
-      it2->second->UpdateDesiredEncryptionKey(
+      entity->UpdateDesiredEncryptionKey(
           data_type_state_.encryption_key_name());
     }