[Sync] ModelTypeStore::ReadMetadataCallback now returns a MetadataBatch

components/sync/model_impl/model_type_store_impl.h

Index: components/sync/model_impl/model_type_store_impl.h
diff --git a/components/sync/model_impl/model_type_store_impl.h b/components/sync/model_impl/model_type_store_impl.h
index 025b9f114e884fa97f9967668f4751ddcc37be8d..172cf09707ec40e11e5ba29cb80aad39f84c1c86 100644
--- a/components/sync/model_impl/model_type_store_impl.h
+++ b/components/sync/model_impl/model_type_store_impl.h
@@ -103,6 +103,12 @@ class ModelTypeStoreImpl : public ModelTypeStore, public base::NonThreadSafe {
   void WriteModificationsDone(const CallbackWithResult& callback,
                               Result result);
 
+  // Helper function to create a SyncError with message |msg|.
+  SyncError MakeSyncError(const std::string& msg);
+
+  // The model type using this store.
+  const ModelType type_;

[skym, 2016/11/05 00:45:42]

What do we think about removing
data_prefix_/metadata_prefix_/global_metadata_key_ constants and making the
functions that create them non-static now that we hang onto the type?

[maxbogue, 2016/11/07 17:03:39]

Unsure; I don't have a good sense of the cost of string operations in critical
paths, though I guess nothing in sync is /that/ critical of a path...

Either way I'd rather do it in a followup.

+
   // Backend should be deleted on backend thread.
   // To accomplish this store's dtor posts task to backend thread passing
   // backend ownership to task parameter.