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

ui/views/examples/text_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/text_example.h"
 
 #include "base/utf_string_conversions.h"
 #include "ui/base/resource/resource_bundle.h"
 #include "ui/gfx/canvas.h"
 #include "ui/gfx/canvas_skia.h"
 #include "ui/views/examples/example_combobox_model.h"
 #include "ui/views/layout/grid_layout.h"
 #include "views/controls/button/checkbox.h"
+#include "views/controls/combobox/combobox.h"
 #include "views/controls/label.h"
 #include "views/view.h"
 
 namespace {
 
 const char kShortText[] = "Batman";
 const char kMediumText[] = "The quick brown fox jumps over the lazy dog.";
 const char kLongText[] =
     "Lorem ipsum dolor sit amet, consectetur adipisicing elit, sed do eiusmod "
     "tempor incididunt ut labore et dolore magna aliqua. Ut enim ad minim "
@@ skipping 295 matching lines @@
       case 2:
         text_flags |= gfx::Canvas::HIDE_PREFIX;
         break;
     }
   }
   text_view_->set_text_flags(text_flags);
   text_view_->SchedulePaint();
 }
 
 }  // namespace examples