views: Extract Widget observer into its own header file.

ash/launcher/overflow_bubble.h

Index: ash/launcher/overflow_bubble.h
diff --git a/ash/launcher/overflow_bubble.h b/ash/launcher/overflow_bubble.h
index aa2d0e32f36a445cbf78a2a52b5e4978119fd3dc..88c4d767f842479dd1b4f7607a74e128d808e90a 100644
--- a/ash/launcher/overflow_bubble.h
+++ b/ash/launcher/overflow_bubble.h
@@ -8,7 +8,7 @@
 #include "ash/wm/shelf_types.h"
 #include "base/basictypes.h"
 #include "base/compiler_specific.h"
-#include "ui/views/widget/widget.h"
+#include "ui/views/widget/widget_observer.h"
 
 namespace views {
 class View;
@@ -24,7 +24,7 @@ namespace internal {
 class LauncherView;
 
 // OverflowBubble displays an overflow bubble.
-class OverflowBubble : public views::Widget::Observer {
+class OverflowBubble : public views::WidgetObserver {
  public:
   OverflowBubble();
   virtual ~OverflowBubble();
@@ -40,7 +40,7 @@ class OverflowBubble : public views::Widget::Observer {
   bool IsShowing() const { return !!bubble_; }
 
  private:
-  // views::Widget::Observer overrides:
+  // Overridden from views::WidgetObserver:
   virtual void OnWidgetClosing(views::Widget* widget) OVERRIDE;
 
   views::View* bubble_;  // Owned by views hierarchy.