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

views/controls/combobox/combobox.h

Index: views/controls/combobox/combobox.h
diff --git a/views/controls/combobox/combobox.h b/views/controls/combobox/combobox.h
index a58268897207aac98d0070b2a8a6b247ba507fdc..85791226b1b03f0f710ab89d9e2a83f62468f6a1 100644
--- a/views/controls/combobox/combobox.h
+++ b/views/controls/combobox/combobox.h
@@ -18,29 +18,20 @@ class ComboboxModel;
 
 namespace views {
 
+class ComboboxListener;
+
 // A non-editable combo-box (aka a drop-down list)
 class VIEWS_EXPORT Combobox : public View {
  public:
   // The combobox's class name.
   static const char kViewClassName[];
 
-  class Listener {
-   public:
-    // This is invoked once the selected item changed.
-    virtual void ItemChanged(Combobox* combo_box,
-                             int prev_index,
-                             int new_index) = 0;
-
-   protected:
-    virtual ~Listener() {}
-  };
-
   // |model| is not owned by the combo box.
   explicit Combobox(ui::ComboboxModel* model);
   virtual ~Combobox();
 
   // Register |listener| for item change events.
-  void set_listener(Listener* listener) {
+  void set_listener(ComboboxListener* listener) {
     listener_ = listener;
   }
 
@@ -94,8 +85,8 @@ class VIEWS_EXPORT Combobox : public View {
   // Our model.
   ui::ComboboxModel* model_;
 
-  // Item change listener.
-  Listener* listener_;
+  // The combobox's listener. Notified when the selected item change.
+  ComboboxListener* listener_;
 
   // The current selection.
   int selected_item_;