Add View::AddedToWidget and RemovedFromWidget.

ui/views/view.h

Index: ui/views/view.h
diff --git a/ui/views/view.h b/ui/views/view.h
index fcbee85c1bf2155eef7639c5b96e75c6980c1b4f..2b8c67a42c42094e4c7989b558ab6104ab3a733c 100644
--- a/ui/views/view.h
+++ b/ui/views/view.h
@@ -1096,10 +1096,11 @@ class VIEWS_EXPORT View : public ui::LayerDelegate,
   // view and all parents.
   //
   // When a view is added, a notification is sent to the view, all its
-  // parents, and all its children (and grand children)
+  // parents, and all its children (and grand children).
   //
-  // Default implementation does nothing. Override to perform operations
-  // required when a view is added or removed from a view hierarchy
+  // Default implementation calls AddedToWidget and RemovedFromWidget.
+  // Override to perform operations required when a view is added or
+  // removed from a view hierarchy.
   //
   // Refer to comments in struct |ViewHierarchyChangedDetails| for |details|.
   virtual void ViewHierarchyChanged(const ViewHierarchyChangedDetails& details);
@@ -1116,6 +1117,14 @@ class VIEWS_EXPORT View : public ui::LayerDelegate,
   // FocusManager manages this view.
   virtual void NativeViewHierarchyChanged();
 
+  // This method is invoked for a view when it is attached to a hierarchy with
+  // a widget, i.e. GetWidget() starts returning a non-null result.
+  virtual void AddedToWidget();
+
+  // This method is invoked for a view when it is removed from a hierarchy with
+  // a widget.
+  virtual void RemovedFromWidget();
+
   // Painting ------------------------------------------------------------------
 
   // Responsible for calling Paint() on child Views. Override to control the
@@ -1491,6 +1500,9 @@ class VIEWS_EXPORT View : public ui::LayerDelegate,
   // This view's children.
   Views children_;
 
+  // The widget this view is (possibily indirectly) attached to.
+  Widget* attached_widget_;
+
 #if DCHECK_IS_ON()
   // True while iterating over |children_|. Used to detect and DCHECK when
   // |children_| is mutated during iteration.