[USS] Add SyncMetadataStore interface

components/sync/model/metadata_change_list.h

Index: components/sync/model/metadata_change_list.h
diff --git a/components/sync/model/metadata_change_list.h b/components/sync/model/metadata_change_list.h
index 8af29f05610057d4056cc906f86198be0e732738..f6c17cf0c54794f43a7c86d9a88dc4f420b71e12 100644
--- a/components/sync/model/metadata_change_list.h
+++ b/components/sync/model/metadata_change_list.h
@@ -7,6 +7,9 @@
 
 #include <string>
 
+#include "base/optional.h"
+#include "components/sync/model/model_error.h"
+
 namespace sync_pb {
 class EntityMetadata;
 class ModelTypeState;
@@ -43,6 +46,9 @@ class MetadataChangeList {
 
   // Requests metadata entry to be cleared from the storage.
   virtual void ClearMetadata(const std::string& storage_key) = 0;
+
+  // Returns the value of ModelError and unsets it.
+  virtual base::Optional<syncer::ModelError> TakeError() = 0;

[skym, 2017/04/06 23:01:58]

What was the decision yesterday about returning errors from MCLs? Is it just
that we'll talk about it next week again?

[pavely, 2017/04/06 23:36:02]

The decision was that we don't change the pattern in this CL, instead consider
this as a separate change. If we decide to return errors from individual
functions I think we'll need to refactor SMTP's functions to handle these
errors.

[Gang Wu, 2017/04/21 05:28:03]

Done.

 };
 
 }  // namespace syncer