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

components/sync/engine_impl/cycle/sync_cycle.h

Index: components/sync/engine_impl/cycle/sync_cycle.h
diff --git a/components/sync/engine_impl/cycle/sync_cycle.h b/components/sync/engine_impl/cycle/sync_cycle.h
index 2c43e1812dcc51380e55e8125741842d83b66bb0..5349f2e9132dd80ad1ad070ef05fb3b4d60d926c 100644
--- a/components/sync/engine_impl/cycle/sync_cycle.h
+++ b/components/sync/engine_impl/cycle/sync_cycle.h
@@ -47,6 +47,9 @@ class SyncCycle {
     virtual void OnTypesThrottled(ModelTypeSet types,
                                   const base::TimeDelta& throttle_duration) = 0;
 
+    // Some of the client's types were backed off.
+    virtual void OnTypesBackedOff(ModelTypeSet types) = 0;
+
     // Silenced intervals can be out of phase with individual cycles, so the
     // delegate is the only thing that can give an authoritative answer for
     // "is syncing silenced right now". This shouldn't be necessary very often