views: Factor out Listener from Combobox into its own header file.

ui/views/examples/combobox_example.cc

 // Copyright (c) 2011 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/combobox_example.h"
 
 #include "base/stringprintf.h"
 #include "base/utf_string_conversions.h"
 #include "ui/base/models/combobox_model.h"
 #include "ui/views/layout/fill_layout.h"
+#include "views/controls/combobox/combobox.h"
 
 namespace {
 
 // An sample combobox model that generates list of "Item <index>".
 class ComboboxModelExample : public ui::ComboboxModel {
  public:
   ComboboxModelExample() {}
   virtual ~ComboboxModelExample() {}
 
   // Overridden from ui::ComboboxModel:
@@ skipping 27 matching lines @@
 }
 
 void ComboboxExample::ItemChanged(views::Combobox* combo_box,
                                   int prev_index,
                                   int new_index) {
   PrintStatus("Selected: index=%d, label=%s",
       new_index, UTF16ToUTF8(combo_box->model()->GetItemAt(new_index)).c_str());
 }
 
 }  // namespace examples