views: Extract Widget observer into its own header file.

chrome/browser/chromeos/status/data_promo_notification.h

Index: chrome/browser/chromeos/status/data_promo_notification.h
diff --git a/chrome/browser/chromeos/status/data_promo_notification.h b/chrome/browser/chromeos/status/data_promo_notification.h
index 6eaa4b074a524a418166eadc5d09efb9f01b7078..d45384b39dc9bea43e8a76060fe9a45c0f825193 100644
--- a/chrome/browser/chromeos/status/data_promo_notification.h
+++ b/chrome/browser/chromeos/status/data_promo_notification.h
@@ -7,20 +7,20 @@
 
 #include "base/basictypes.h"
 #include "base/memory/weak_ptr.h"
-#include "ui/views/widget/widget.h"
+#include "ui/views/widget/widget_observer.h"
 
 class PrefService;
 
 namespace views {
 class View;
-}  // namespace views
+}
 
 namespace chromeos {
 class MessageBubble;
 class MessageBubbleLinkListener;
 class NetworkLibrary;
 
-class DataPromoNotification : public views::Widget::Observer {
+class DataPromoNotification : public views::WidgetObserver {
  public:
   DataPromoNotification();
   virtual ~DataPromoNotification();
@@ -40,7 +40,7 @@ class DataPromoNotification : public views::Widget::Observer {
   void CloseNotification();
 
  private:
-  // Overridden from views::Widget::Observer.
+  // Overridden from views::WidgetObserver:
   virtual void OnWidgetClosing(views::Widget* widget) OVERRIDE;
 
   // Notification bubble for 3G promo.