cleanup: make WidgetClosingObserver as test support

ui/views/test/widget_test.h

Index: ui/views/test/widget_test.h
diff --git a/ui/views/test/widget_test.h b/ui/views/test/widget_test.h
index 62be1b7e4c029723046169dedc36e728e442677b..a796a26f3c9a5bb9fb777d8c254fa2853de9e5c2 100644
--- a/ui/views/test/widget_test.h
+++ b/ui/views/test/widget_test.h
@@ -177,6 +177,24 @@ class WidgetActivationWaiter : public WidgetObserver {
   DISALLOW_COPY_AND_ASSIGN(WidgetActivationWaiter);
 };
 
+// Use in tests to provide functionality to observe the widget passed in the
+// constructor for the widget closing event.
+class WidgetClosingObserver : public WidgetObserver {
+ public:
+  explicit WidgetClosingObserver(Widget* widget);
+  ~WidgetClosingObserver() override;
+
+  bool widget_closed() const { return !widget_; }
+
+ private:
+  // views::WidgetObserver override:
+  void OnWidgetClosing(Widget* widget) override;
+
+  Widget* widget_;
+
+  DISALLOW_COPY_AND_ASSIGN(WidgetClosingObserver);
+};
+
 }  // namespace test
 }  // namespace views