Removes SimpleMenuModel::FlipIndex. It was only used by

ui/base/models/simple_menu_model.h

Index: ui/base/models/simple_menu_model.h
diff --git a/ui/base/models/simple_menu_model.h b/ui/base/models/simple_menu_model.h
index 309b528e84e2c51a9bfbd0dda4aada734e81f16f..8aa9d9b8f887a9f245391e91f3a4b911aa5cc21c 100644
--- a/ui/base/models/simple_menu_model.h
+++ b/ui/base/models/simple_menu_model.h
@@ -142,13 +142,6 @@ class UI_EXPORT SimpleMenuModel : public MenuModel {
   virtual MenuModelDelegate* GetMenuModelDelegate() const OVERRIDE;
 
  protected:
-  // Some variants of this model (SystemMenuModel) relies on items to be
-  // inserted backwards. This is counter-intuitive for the API, so rather than
-  // forcing customers to insert things backwards, we return the indices
-  // backwards instead. That's what this method is for. By default, it just
-  // returns what it's passed.
-  virtual int FlipIndex(int index) const;
-
   void set_delegate(Delegate* delegate) { delegate_ = delegate; }
   Delegate* delegate() { return delegate_; }
 
@@ -157,7 +150,7 @@ class UI_EXPORT SimpleMenuModel : public MenuModel {
 
   typedef std::vector<Item> ItemVector;
 
-  // Caller needs to call FlipIndex() if necessary. Returns |index|.
+  // Returns |index|.
   int ValidateItemIndex(int index) const;
 
   // Functions for inserting items into |items_|.