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(Window* window)
+    : window_(window), destroyed_(false) {
+  DCHECK(window_);
+  window_->AddObserver(this);
+}
+
+WindowDestructionObserver::~WindowDestructionObserver() {
+  if (!destroyed_)
+    window_->RemoveObserver(this);
+}
+
+void WindowDestructionObserver::OnWindowDestroyed(Window* window) {
+  if (window == window_)
+    destroyed_ = true;
+}
+
+} // namespace aura