views: Extract Widget observer into its own header file.

ash/system/tray/system_tray_bubble.h

Index: ash/system/tray/system_tray_bubble.h
diff --git a/ash/system/tray/system_tray_bubble.h b/ash/system/tray/system_tray_bubble.h
index 269840c11f74982b120f4c27ec0fa1976cc840bc..827a12936325f9e931d8ebccb9d2d776da7d50b9 100644
--- a/ash/system/tray/system_tray_bubble.h
+++ b/ash/system/tray/system_tray_bubble.h
@@ -10,7 +10,7 @@
 #include "ash/wm/shelf_types.h"
 #include "base/base_export.h"
 #include "base/timer.h"
-#include "ui/views/widget/widget.h"
+#include "ui/views/widget/widget_observer.h"
 
 #include <vector>
 
@@ -22,7 +22,7 @@ class SystemTrayItem;
 namespace internal {
 
 class SystemTrayBubble : public TrayBubbleView::Host,
-                         public views::Widget::Observer {
+                         public views::WidgetObserver {
  public:
   enum BubbleType {
     BUBBLE_TYPE_DEFAULT,
@@ -80,7 +80,7 @@ class SystemTrayBubble : public TrayBubbleView::Host,
  private:
   void CreateItemViews(user::LoginStatus login_status);
 
-  // Overridden from views::Widget::Observer.
+  // Overridden from views::WidgetObserver:
   virtual void OnWidgetClosing(views::Widget* widget) OVERRIDE;
 
   ash::SystemTray* tray_;