cleanup: make WidgetClosingObserver as test support

ui/views/test/widget_test.cc

Index: ui/views/test/widget_test.cc
diff --git a/ui/views/test/widget_test.cc b/ui/views/test/widget_test.cc
index 457a63da806ba5eb6446b3b1e701611b12c4dc10..a96a9db970a7e626486ba275afa1d43365fe2a1d 100644
--- a/ui/views/test/widget_test.cc
+++ b/ui/views/test/widget_test.cc
@@ -68,7 +68,7 @@ Widget* WidgetTest::CreateChildNativeWidgetWithParent(Widget* parent) {
 }
 
 Widget* WidgetTest::CreateChildNativeWidget() {
-  return CreateChildNativeWidgetWithParent(NULL);
+  return CreateChildNativeWidgetWithParent(nullptr);
 }
 
 Widget* WidgetTest::CreateNativeDesktopWidget() {
@@ -177,5 +177,20 @@ void WidgetActivationWaiter::OnWidgetActivationChanged(Widget* widget,
     run_loop_.Quit();
 }
 
+WidgetClosingObserver::WidgetClosingObserver(Widget* widget) : widget_(widget) {
+  widget_->AddObserver(this);
+}
+
+WidgetClosingObserver::~WidgetClosingObserver() {
+  if (widget_)
+    widget_->RemoveObserver(this);
+}
+
+void WidgetClosingObserver::OnWidgetClosing(Widget* widget) {
+  DCHECK_EQ(widget_, widget);
+  widget_->RemoveObserver(this);
+  widget_ = nullptr;
+}
+
 }  // namespace test
 }  // namespace views