Fix crash when user closes window prior to the "Confirm Install" prompt showing

chrome/browser/ui/aura/native_window_tracker_aura.cc

+// Copyright 2014 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 "chrome/browser/ui/aura/native_window_tracker_aura.h"
+
+#include "ui/aura/window.h"
+
+NativeWindowTrackerAura::NativeWindowTrackerAura(
+    gfx::NativeWindow window)
+    : window_(window) {
+  window->AddObserver(this);
+}
+
+NativeWindowTrackerAura::~NativeWindowTrackerAura() {
+  if (window_)
+    window_->RemoveObserver(this);
+}
+
+bool NativeWindowTrackerAura::WasNativeWindowClosed() const {
+  return window_ == nullptr;
+}
+
+void NativeWindowTrackerAura::OnWindowDestroying(
+    aura::Window* window) {
+  window_->RemoveObserver(this);
+  window_ = nullptr;
+}
+
+// static
+scoped_ptr<NativeWindowTracker> NativeWindowTracker::Create(
+    gfx::NativeWindow window) {
+  return scoped_ptr<NativeWindowTracker>(new NativeWindowTrackerAura(window));
+}