views: Extract Widget observer into its own header file.

ash/touch/touch_observer_hud.h

Index: ash/touch/touch_observer_hud.h
diff --git a/ash/touch/touch_observer_hud.h b/ash/touch/touch_observer_hud.h
index 9297f83f4d0c910412a4eaef9dddd92a4cd71d76..9c8ca6ec3ba1bd26436112638917dc65d5f4fd22 100644
--- a/ash/touch/touch_observer_hud.h
+++ b/ash/touch/touch_observer_hud.h
@@ -8,7 +8,7 @@
 #include "ash/shell.h"
 #include "ui/aura/event_filter.h"
 #include "ui/gfx/point.h"
-#include "ui/views/widget/widget.h"
+#include "ui/views/widget/widget_observer.h"
 
 namespace aura {
 class MouseEvent;
@@ -28,7 +28,7 @@ class TouchHudCanvas;
 
 // An event filter which handles system level gesture events.
 class TouchObserverHUD : public aura::EventFilter,
-                         public views::Widget::Observer {
+                         public views::WidgetObserver {
  public:
   TouchObserverHUD();
   virtual ~TouchObserverHUD();
@@ -47,7 +47,7 @@ class TouchObserverHUD : public aura::EventFilter,
       aura::Window* target,
       aura::GestureEvent* event) OVERRIDE;
 
-  // Overridden from views::Widget::Observer.
+  // Overridden from views::WidgetObserver:
   virtual void OnWidgetClosing(views::Widget* widget) OVERRIDE;
 
   static const int kMaxTouchPoints = 32;