Verifies WindowClosing() is invoked only once when a window is

ui/views/widget/desktop_aura/desktop_native_widget_aura.cc

 // Copyright (c) 2012 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/widget/desktop_aura/desktop_native_widget_aura.h"
 
 #include "base/bind.h"
 #include "ui/aura/client/stacking_client.h"
 #include "ui/aura/focus_manager.h"
 #include "ui/aura/root_window.h"
@@ skipping 505 matching lines @@
 
 void DesktopNativeWidgetAura::OnPaint(gfx::Canvas* canvas) {
   native_widget_delegate_->OnNativeWidgetPaint(canvas);
 }
 
 void DesktopNativeWidgetAura::OnDeviceScaleFactorChanged(
     float device_scale_factor) {
 }
 
 void DesktopNativeWidgetAura::OnWindowDestroying() {
-  native_widget_delegate_->OnNativeWidgetDestroying();
+  // The DesktopRootWindowHost implementation sends OnNativeWidgetDestroying().
 }
 
 void DesktopNativeWidgetAura::OnWindowDestroyed() {
   window_ = NULL;
   native_widget_delegate_->OnNativeWidgetDestroyed();
   if (ownership_ == Widget::InitParams::NATIVE_WIDGET_OWNS_WIDGET)
     delete this;
 }
 
 void DesktopNativeWidgetAura::OnWindowTargetVisibilityChanged(bool visible) {
@@ skipping 176 matching lines @@
   drop_helper_->OnDragExit();
 }
 
 int DesktopNativeWidgetAura::OnPerformDrop(const ui::DropTargetEvent& event) {
   DCHECK(drop_helper_.get() != NULL);
   return drop_helper_->OnDrop(event.data(), event.location(),
       last_drop_operation_);
 }
 
 }  // namespace views