[Sync] Introduce ModelError for USS error handling.

components/sync/model/model_type_change_processor.h

Index: components/sync/model/model_type_change_processor.h
diff --git a/components/sync/model/model_type_change_processor.h b/components/sync/model/model_type_change_processor.h
index b001832855e7cbb246aaa2463c023b32ab8a4bdb..838449815b13ca75cf71e38a46aedd2ddf62bf30 100644
--- a/components/sync/model/model_type_change_processor.h
+++ b/components/sync/model/model_type_change_processor.h
@@ -8,11 +8,12 @@
 #include <memory>
 #include <string>
 
+#include "base/location.h"
 #include "components/sync/base/model_type.h"
 #include "components/sync/engine/activation_context.h"
 #include "components/sync/model/data_type_error_handler.h"
 #include "components/sync/model/entity_data.h"
-#include "components/sync/model/sync_error_factory.h"
+#include "components/sync/model/model_error.h"
 
 namespace syncer {
 
@@ -23,7 +24,7 @@ class SyncError;
 
 // Interface used by the ModelTypeSyncBridge to inform sync of local
 // changes.
-class ModelTypeChangeProcessor : public SyncErrorFactory {
+class ModelTypeChangeProcessor {
  public:
   typedef base::Callback<void(SyncError, std::unique_ptr<ActivationContext>)>
       StartCallback;
@@ -34,7 +35,7 @@ class ModelTypeChangeProcessor : public SyncErrorFactory {
       ModelTypeSyncBridge* bridge);
 
   ModelTypeChangeProcessor();
-  ~ModelTypeChangeProcessor() override;
+  virtual ~ModelTypeChangeProcessor();
 
   // Inform the processor of a new or updated entity. The |entity_data| param
   // does not need to be fully set, but it should at least have specifics and
@@ -48,10 +49,10 @@ class ModelTypeChangeProcessor : public SyncErrorFactory {
   virtual void Delete(const std::string& storage_key,
                       MetadataChangeList* metadata_change_list) = 0;
 
-  // Accept the initial sync metadata loaded by the bridge. This should be
-  // called as soon as the metadata is available to the bridge.
-  virtual void OnMetadataLoaded(SyncError error,
-                                std::unique_ptr<MetadataBatch> batch) = 0;
+  // Accept the initial sync metadata loaded by the bridge. This must be called
+  // by the bridge for syncing to begin for this model type. If an error occurs,
+  // call ReportError instead of this.
+  virtual void OnMetadataLoaded(std::unique_ptr<MetadataBatch> batch) = 0;
 
   // Indicates that sync wants to connect a sync worker to this processor. Once
   // the processor has metadata from the bridge, it will pass the info needed
@@ -71,6 +72,17 @@ class ModelTypeChangeProcessor : public SyncErrorFactory {
   // currently up to date and accurately tracking the model type's data. If
   // false, calls to Put and Delete will no-op and can be omitted by bridge.
   virtual bool IsTrackingMetadata() = 0;
+
+  // Report an error in the model to sync. Should be called for any persistence
+  // or consistency error the bridge encounters outside of a method that allows
+  // returning a ModelError directly. Outstanding callbacks are not expected to
+  // be called after an error. This will result in sync being temporarily
+  // disabled for the model type (generally until the next restart).
+  virtual void ReportError(const ModelError& error) = 0;
+
+  // A convenience form of the above.
+  virtual void ReportError(const tracked_objects::Location& location,
+                           const std::string& message) = 0;
 };
 
 }  // namespace syncer