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..df4832741677a245b32107d7f837cb87ec4687b8 100644
--- a/ui/views/view.h
+++ b/ui/views/view.h
@@ -1116,6 +1116,14 @@ 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.

[sky, 2017/03/01 20:26:51]

Clarify this is also invoked if a view is moved to a different widget.

[msimonides, 2017/03/02 07:48:56]

Done.

+  virtual void RemovedFromWidget();
+
   // Painting ------------------------------------------------------------------
 
   // Responsible for calling Paint() on child Views. Override to control the
@@ -1298,10 +1306,16 @@ class VIEWS_EXPORT View : public ui::LayerDelegate,
   // |old_parent| is the original parent of the View that was removed.
   // If |new_parent| is not NULL, the View that was removed will be reparented
   // to |new_parent| after the remove operation.
-  void PropagateRemoveNotifications(View* old_parent, View* new_parent);
+  // If is_removed_from_widget is true, calls RemovedFromWidget for all
+  // children.
+  void PropagateRemoveNotifications(View* old_parent,
+                                    View* new_parent,
+                                    bool is_removed_from_widget);
 
   // Call ViewHierarchyChanged() for all children.
-  void PropagateAddNotifications(const ViewHierarchyChangedDetails& details);
+  // If is_added_to_widget is true, calls AddedToWidget for all children.
+  void PropagateAddNotifications(const ViewHierarchyChangedDetails& details,
+                                 bool is_added_to_widget);
 
   // Propagates NativeViewHierarchyChanged() notification through all the
   // children.