sync: Move migration signal out of snapshot

sync/engine/sync_engine_event_listener.h

Index: sync/engine/sync_engine_event_listener.h
diff --git a/sync/engine/sync_engine_event_listener.h b/sync/engine/sync_engine_event_listener.h
index 8e5c3d5684a2bce4d18d6255e47b4d98263b7b7e..35fc05e8ce956db59adccf711add2f0b6c627efd 100644
--- a/sync/engine/sync_engine_event_listener.h
+++ b/sync/engine/sync_engine_event_listener.h
@@ -33,6 +33,9 @@ class SYNC_EXPORT_PRIVATE SyncEngineEventListener {
   // This event is sent when types are throttled or unthrottled.
   virtual void OnThrottledTypesChanged(ModelTypeSet throttled_types) = 0;
 
+  // This event is sent when the server requests a migration.
+  virtual void OnMigrationRequested(ModelTypeSet migration_types) = 0;
+
  protected:
   virtual ~SyncEngineEventListener();
 };