Add View::AddedToWidget and RemovedFromWidget.

ui/views/view.cc

Index: ui/views/view.cc
diff --git a/ui/views/view.cc b/ui/views/view.cc
index 04c5f2fae25957c3ca3b706654e263fc8b3c6638..dfe0cc98741cfe36f7ad893aff37f54def0a7170 100644
--- a/ui/views/view.cc
+++ b/ui/views/view.cc
@@ -1501,6 +1501,18 @@ void View::OnVisibleBoundsChanged() {
 // Tree operations -------------------------------------------------------------
 
 void View::ViewHierarchyChanged(const ViewHierarchyChangedDetails& details) {
+  if (details.is_add) {
+    const auto widget = GetWidget();
+    if (widget != nullptr && widget != attached_widget_) {
+      attached_widget_ = widget;
+      AddedToWidget();
+    }
+  } else if (details.child == this) {
+    if (attached_widget_) {
+      RemovedFromWidget();
+      attached_widget_ = nullptr;
+    }
+  }
 }
 
 void View::VisibilityChanged(View* starting_from, bool is_visible) {
@@ -1516,6 +1528,10 @@ void View::NativeViewHierarchyChanged() {
   }
 }
 
+void View::AddedToWidget() {}
+
+void View::RemovedFromWidget() {}
+
 // Painting --------------------------------------------------------------------
 
 void View::PaintChildren(const ui::PaintContext& context) {