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

ui/views/examples/example_combobox_model.cc

-// 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.
 
 #include "ui/views/examples/example_combobox_model.h"
 
-#include "base/stringprintf.h"
 #include "base/utf_string_conversions.h"
-#include "ui/base/models/combobox_model.h"
 
 namespace views {
 namespace examples {
 
 ExampleComboboxModel::ExampleComboboxModel(const char** strings, int count)
     : strings_(strings), count_(count) {
 }
 
 ExampleComboboxModel::~ExampleComboboxModel() {
 }
 
-int ExampleComboboxModel::GetItemCount() {
+int ExampleComboboxModel::GetItemCount() const {
   return count_;
 }
 
 string16 ExampleComboboxModel::GetItemAt(int index) {
   return ASCIIToUTF16(strings_[index]);
 }
 
 }  // namespace examples
 }  // namespace views