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

ui/views/examples/combobox_example.h

 // 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.
 
 #ifndef UI_VIEWS_EXAMPLES_COMBOBOX_EXAMPLE_H_
 #define UI_VIEWS_EXAMPLES_COMBOBOX_EXAMPLE_H_
 #pragma once
 
 #include "base/basictypes.h"
 #include "base/compiler_specific.h"
 #include "ui/views/examples/example_base.h"
-#include "views/controls/combobox/combobox.h"
+#include "views/controls/combobox/combobox_listener.h"
 
 namespace examples {
 
 class ComboboxExample : public ExampleBase,
-                        public views::Combobox::Listener {
+                        public views::ComboboxListener {
  public:
   explicit ComboboxExample(ExamplesMain* main);
   virtual ~ComboboxExample();
 
   // Overridden from ExampleBase:
   virtual void CreateExampleView(views::View* container) OVERRIDE;
 
  private:
-  // Overridden from views::Combobox::Listener:
+  // Overridden from views::ComboboxListener:
   virtual void ItemChanged(views::Combobox* combo_box,
                            int prev_index,
                            int new_index) OVERRIDE;
 
   views::Combobox* combobox_;
 
   DISALLOW_COPY_AND_ASSIGN(ComboboxExample);
 };
 
 }  // namespace examples
 
 #endif  // UI_VIEWS_EXAMPLES_COMBOBOX_EXAMPLE_H_