[Sync] Convert NonBlockingDataTypeController to be single-thread.

components/sync/api/model_type_service.h

Index: components/sync/api/model_type_service.h
diff --git a/components/sync/api/model_type_service.h b/components/sync/api/model_type_service.h
index 49a56e7ebcf7e5299959ddccd3214a7b87141c47..c4a8779a0c0cc308fc935bf795757dc59166a9c0 100644
--- a/components/sync/api/model_type_service.h
+++ b/components/sync/api/model_type_service.h
@@ -10,6 +10,7 @@
 #include <vector>
 
 #include "base/callback.h"
+#include "base/memory/weak_ptr.h"
 #include "components/sync/api/conflict_resolution.h"
 #include "components/sync/api/entity_change.h"
 #include "components/sync/api/entity_data.h"
@@ -29,7 +30,7 @@ class MetadataChangeList;
 // Interface implemented by model types to receive updates from sync via the
 // SharedModelTypeProcessor. Provides a way for sync to update the data and
 // metadata for entities, as well as the model type state.
-class ModelTypeService {
+class ModelTypeService : public base::SupportsWeakPtr<ModelTypeService> {
  public:
   typedef base::Callback<void(syncer::SyncError, std::unique_ptr<DataBatch>)>
       DataCallback;