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..ed05c92419b5202901bec530942c186e63cc76e1 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.
+  virtual void RemovedFromWidget();
+
   // Painting ------------------------------------------------------------------
 
   // Responsible for calling Paint() on child Views. Override to control the
@@ -1315,6 +1323,12 @@ class VIEWS_EXPORT View : public ui::LayerDelegate,
   // Invokes OnNativeThemeChanged() on this and all descendants.
   void PropagateNativeThemeChanged(const ui::NativeTheme* theme);
 
+  // Invokes AddedToWidget() on this and all descendants.
+  void PropagateAddedToWidget();
+
+  // Invokes RemovedFromWidget() on this and all descendants.
+  void PropagateRemovedFromWidget();
+
   // Size and disposition ------------------------------------------------------
 
   // Call VisibilityChanged() recursively for all children.