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

ui/views/examples/example_combobox_model.h

-// Copyright (c) 2011 The Chromium Authors. All rights reserved.
+// 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_VIEWS_EXAMPLES_EXAMPLE_COMBOBOX_MODEL_H_
 #define UI_VIEWS_EXAMPLES_EXAMPLE_COMBOBOX_MODEL_H_
 #pragma once
 
+#include "base/basictypes.h"
 #include "base/compiler_specific.h"
 #include "ui/base/models/combobox_model.h"
 
 namespace views {
 namespace examples {
 
 class ExampleComboboxModel : public ui::ComboboxModel {
  public:
   ExampleComboboxModel(const char** strings, int count);
   virtual ~ExampleComboboxModel();
 
   // Overridden from ui::ComboboxModel:
-  virtual int GetItemCount() OVERRIDE;
+  virtual int GetItemCount() const OVERRIDE;
   virtual string16 GetItemAt(int index) OVERRIDE;
 
  private:
   const char** strings_;
   int count_;
 
   DISALLOW_COPY_AND_ASSIGN(ExampleComboboxModel);
 };
 
 }  // namespace examples
 }  // namespace views
 
 #endif  // UI_VIEWS_EXAMPLES_EXAMPLE_COMBOBOX_MODEL_H_