ui/base/models: Make ComboboxModel::GetItemCount() a constant function.

ui/base/models/combobox_model.h

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef UI_BASE_MODELS_COMBOBOX_MODEL_H_
 #define UI_BASE_MODELS_COMBOBOX_MODEL_H_
 #pragma once
 
 #include "base/string16.h"
 
 namespace ui {
 
 // A data model for a combo box.
 class ComboboxModel {
  public:
   // Returns the number of items in the combo box.
-  virtual int GetItemCount() = 0;
+  virtual int GetItemCount() const = 0;
 
   // Returns the string at the specified index.
   virtual string16 GetItemAt(int index) = 0;
 
  protected:
   virtual ~ComboboxModel() {}
 };
 
 }  // namespace ui
 
 #endif  // UI_BASE_MODELS_COMBOBOX_MODEL_H_