Observe for window's (self) deletion during activation/focus change

ui/aura/window_destruction_observer.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/aura/window_destruction_observer.h"
+
+namespace aura {
+
+WindowDestructionObserver::WindowDestructionObserver() {
+}
+
+WindowDestructionObserver::~WindowDestructionObserver() {
+}
+
+bool WindowDestructionObserver::was_destroyed(const Window* window) const {
+  return destroyed_windows_.find(window) != destroyed_windows_.end();
+}
+
+void WindowDestructionObserver::OnWindowDestroyed(Window* window) {
+  destroyed_windows_.insert(window);
+}
+
+} // namespace aura