[Sync] Sync client should to exponential backoff when receive partial failure

components/sync/engine_impl/sync_engine_event_listener.h

Index: components/sync/engine_impl/sync_engine_event_listener.h
diff --git a/components/sync/engine_impl/sync_engine_event_listener.h b/components/sync/engine_impl/sync_engine_event_listener.h
index 240a05474e18826ec739ef3bfff86ab974c21626..d9f4393469a27f580c87ac7e28d2557cdb8c7e98 100644
--- a/components/sync/engine_impl/sync_engine_event_listener.h
+++ b/components/sync/engine_impl/sync_engine_event_listener.h
@@ -33,6 +33,9 @@ class SyncEngineEventListener {
   // This event is sent when types are throttled or unthrottled.
   virtual void OnThrottledTypesChanged(ModelTypeSet throttled_types) = 0;
 
+  // This event is sent when types are backed off or unbacked off.
+  virtual void OnBackedOffTypesChanged(ModelTypeSet backed_off_types) = 0;
+
   // This event is sent when the server requests a migration.
   virtual void OnMigrationRequested(ModelTypeSet migration_types) = 0;