[Sync] Introduce ModelError for USS error handling.

components/sync/model_impl/shared_model_type_processor.h

Index: components/sync/model_impl/shared_model_type_processor.h
diff --git a/components/sync/model_impl/shared_model_type_processor.h b/components/sync/model_impl/shared_model_type_processor.h
index 65530d4652328cdfaa8871c5ce52b18100e212af..d8ff456b6e0188eefc875912bad658e55b111fa0 100644
--- a/components/sync/model_impl/shared_model_type_processor.h
+++ b/components/sync/model_impl/shared_model_type_processor.h
@@ -58,8 +58,11 @@ class SharedModelTypeProcessor : public ModelTypeProcessor,
                       const StartCallback& callback) override;
   void DisableSync() override;
   bool IsTrackingMetadata() override;
+  void OnUnrecoverableError(const SyncError& error) override;
   SyncError CreateAndUploadError(const tracked_objects::Location& location,
-                                 const std::string& message) override;
+                                 const std::string& message,
+                                 ModelType type) override;
+  std::unique_ptr<DataTypeErrorHandler> Copy() const override;
 
   // ModelTypeProcessor implementation.
   void ConnectSync(std::unique_ptr<CommitQueue> worker) override;
@@ -149,10 +152,10 @@ class SharedModelTypeProcessor : public ModelTypeProcessor,
   SyncError start_error_;
 
   // Indicates whether the metadata has finished loading.
-  bool is_metadata_loaded_;
+  bool is_metadata_loaded_ = false;
 
   // Indicates whether data for any initial pending commits has been loaded.
-  bool is_initial_pending_data_loaded_;
+  bool is_initial_pending_data_loaded_ = false;
 
   // Reference to the CommitQueue.
   //