cleanup: make WidgetClosingObserver as test support

ui/views/test/widget_test.cc

 // Copyright 2013 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "ui/views/test/widget_test.h"
 
 #include "build/build_config.h"
 #include "ui/gfx/native_widget_types.h"
 #include "ui/views/test/native_widget_factory.h"
 #include "ui/views/widget/root_view.h"
@@ skipping 50 matching lines @@
 Widget* WidgetTest::CreateChildNativeWidgetWithParent(Widget* parent) {
   Widget* child = new Widget;
   Widget::InitParams params = CreateParams(Widget::InitParams::TYPE_CONTROL);
   params.parent = parent->GetNativeView();
   child->Init(params);
   child->SetContentsView(new View);
   return child;
 }
 
 Widget* WidgetTest::CreateChildNativeWidget() {
-  return CreateChildNativeWidgetWithParent(NULL);
+  return CreateChildNativeWidgetWithParent(nullptr);
 }
 
 Widget* WidgetTest::CreateNativeDesktopWidget() {
   Widget* widget = new Widget;
   Widget::InitParams params = CreateParams(Widget::InitParams::TYPE_WINDOW);
   params.native_widget =
       CreatePlatformDesktopNativeWidgetImpl(params, widget, nullptr);
   widget->Init(params);
   return widget;
 }
@@ skipping 88 matching lines @@
                                                        bool active) {
   if (active_ != active)
     return;
 
   observed_ = true;
   widget->RemoveObserver(this);
   if (run_loop_.running())
     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