[Sync] Introduce ModelError for USS error handling.

components/sync/model/fake_model_type_change_processor.h

Index: components/sync/model/fake_model_type_change_processor.h
diff --git a/components/sync/model/fake_model_type_change_processor.h b/components/sync/model/fake_model_type_change_processor.h
index bee92efbfecc1bdb5910be15e4f17278abb414be..c98da10f580bc7fd16b77fc7f90084ab4a306ee2 100644
--- a/components/sync/model/fake_model_type_change_processor.h
+++ b/components/sync/model/fake_model_type_change_processor.h
@@ -32,14 +32,20 @@ class FakeModelTypeChangeProcessor : public ModelTypeChangeProcessor {
            MetadataChangeList* metadata_change_list) override;
   void Delete(const std::string& client_tag,
               MetadataChangeList* metadata_change_list) override;
-  void OnMetadataLoaded(SyncError error,
-                        std::unique_ptr<MetadataBatch> batch) override;
+  void OnMetadataLoaded(std::unique_ptr<MetadataBatch> batch) override;
   void OnSyncStarting(std::unique_ptr<DataTypeErrorHandler> error_handler,
                       const StartCallback& callback) override;
   void DisableSync() override;
   bool IsTrackingMetadata() override;
-  SyncError CreateAndUploadError(const tracked_objects::Location& location,
-                                 const std::string& message) override;
+  void ReportError(const ModelError& error) override;
+  void ReportError(const tracked_objects::Location& location,
+                   const std::string& message) override;
+
+  // Returns |error_| and unsets it.
+  ModelError TakeError();
+
+ private:
+  ModelError error_;
 };
 
 }  // namespace syncer