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

ui/views/examples/native_theme_button_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/native_theme_button_example.h"
 
 #include <string>
 
 #include "base/logging.h"
 #include "base/stringprintf.h"
 #include "base/utf_string_conversions.h"
 #include "ui/base/animation/throb_animation.h"
 #include "ui/base/models/combobox_model.h"
 #include "ui/gfx/canvas.h"
 #include "ui/views/examples/example_combobox_model.h"
 #include "ui/views/layout/grid_layout.h"
+#include "views/controls/combobox/combobox.h"
 #include "views/controls/label.h"
 #include "views/native_theme_painter.h"
 
 namespace {
 
 const char* kParts[] = {
     "PushButton",
     "RadioButton",
     "Checkbox",
 };
@@ skipping 219 matching lines @@
 
   layout->AddPaddingRow(0, 8);
 }
 
 void NativeThemeButtonExample::ButtonPressed(views::Button* sender,
                                              const views::Event& event) {
   PrintStatus(button_->MessWithState().c_str());
 }
 
 }  // namespace examples