Add a ListModel::Move.

ui/base/models/list_model_observer.h

Index: ui/base/models/list_model_observer.h
diff --git a/ui/base/models/list_model_observer.h b/ui/base/models/list_model_observer.h
index 4b31cb1c39beb2a3698fbbdc5a386b4094e30210..3f59da979829602173c2b17b47f145ca9371b551 100644
--- a/ui/base/models/list_model_observer.h
+++ b/ui/base/models/list_model_observer.h
@@ -20,6 +20,10 @@ class UI_EXPORT ListModelObserver {
   // removal.
   virtual void ListItemsRemoved(size_t start, size_t count) = 0;
 
+  // Invoked after an item has been moved. See ListModel::Move() for details
+  // of the arguments.
+  virtual void ListItemMoved(size_t index, size_t target_index) = 0;
+
   // Invoked after items has been changed.
   virtual void ListItemsChanged(size_t start, size_t count) = 0;