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

views/controls/combobox/combobox_listener.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 VIEWS_CONTROLS_COMBOBOX_COMBOBOX_LISTENER_H_
+#define VIEWS_CONTROLS_COMBOBOX_COMBOBOX_LISTENER_H_
+#pragma once
+
+namespace views {
+
+class Combobox;
+
+// An interface implemented by an object to let it know that the selected item
+// of a combobox changed.
+class ComboboxListener {
+ public:
+  // This is invoked once the selected item changed.
+  virtual void ItemChanged(Combobox* combo_box,
+                           int prev_index,
+                           int new_index) = 0;
+
+ protected:
+  virtual ~ComboboxListener() {}
+};
+
+}  // namespace views
+
+#endif  // VIEWS_CONTROLS_COMBOBOX_COMBOBOX_LISTENER_H_