[Sync] Introduce ModelError for USS error handling.

components/sync/model/stub_model_type_sync_bridge.h

Index: components/sync/model/stub_model_type_sync_bridge.h
diff --git a/components/sync/model/stub_model_type_sync_bridge.h b/components/sync/model/stub_model_type_sync_bridge.h
index 73fadb9883d9e58e5235bfb58e9128432ddb9b55..38d6f30ff3f9fa8d797cf3613dd483c73ea2c61c 100644
--- a/components/sync/model/stub_model_type_sync_bridge.h
+++ b/components/sync/model/stub_model_type_sync_bridge.h
@@ -22,10 +22,10 @@ class StubModelTypeSyncBridge : public ModelTypeSyncBridge {
   ~StubModelTypeSyncBridge() override;
 
   std::unique_ptr<MetadataChangeList> CreateMetadataChangeList() override;
-  SyncError MergeSyncData(
+  ModelError MergeSyncData(
       std::unique_ptr<MetadataChangeList> metadata_change_list,
       EntityDataMap entity_data_map) override;
-  SyncError ApplySyncChanges(
+  ModelError ApplySyncChanges(
       std::unique_ptr<MetadataChangeList> metadata_change_list,
       EntityChangeList entity_changes) override;
   void GetData(StorageKeyList storage_keys, DataCallback callback) override;