Reland r256396 Add views implementation for AXTreeSource.

ui/views/widget/widget_observer.h

Index: ui/views/widget/widget_observer.h
diff --git a/ui/views/widget/widget_observer.h b/ui/views/widget/widget_observer.h
index fc13afae682799dff309bba9db4f7681b625a4f5..59029ae54e22e2ddbb4c4af9238c63529b52c973 100644
--- a/ui/views/widget/widget_observer.h
+++ b/ui/views/widget/widget_observer.h
@@ -14,6 +14,7 @@ class Rect;
 namespace views {
 
 class Widget;
+class View;
 
 // Observers can listen to various events on the Widgets.
 class VIEWS_EXPORT WidgetObserver {
@@ -45,6 +46,9 @@ class VIEWS_EXPORT WidgetObserver {
   virtual void OnWidgetBoundsChanged(Widget* widget,
                                      const gfx::Rect& new_bounds) {}
 
+  // Called after a descendant view of |widget| is removed.
+  virtual void OnViewRemoved(Widget* widget, View* view) {}
+
  protected:
   virtual ~WidgetObserver() {}
 };