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..529867365d987c4c8bce6daa0c0734ef272aecf0 100644
--- a/ui/views/view.cc
+++ b/ui/views/view.cc
@@ -1516,6 +1516,10 @@ void View::NativeViewHierarchyChanged() {
   }
 }
 
+void View::AddedToWidget() {}
+
+void View::RemovedFromWidget() {}
+
 // Painting --------------------------------------------------------------------
 
 void View::PaintChildren(const ui::PaintContext& context) {
@@ -2012,6 +2016,20 @@ void View::ViewHierarchyChangedImpl(
   }
 
   ViewHierarchyChanged(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;
+    }
+  }
+
   details.parent->needs_layout_ = true;
 }