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

chrome/browser/ui/aura/native_window_tracker_aura.h

+// 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.
+
+#ifndef CHROME_BROWSER_UI_AURA_NATIVE_WINDOW_TRACKER_AURA_H_
+#define CHROME_BROWSER_UI_AURA_NATIVE_WINDOW_TRACKER_AURA_H_
+
+#include "chrome/browser/ui/native_window_tracker.h"
+#include "ui/aura/window_observer.h"
+
+class NativeWindowTrackerAura : public NativeWindowTracker,
+                                public aura::WindowObserver {
+ public:
+  explicit NativeWindowTrackerAura(gfx::NativeWindow window);
+  ~NativeWindowTrackerAura() override;
+
+  // NativeWindowTracker:
+  bool WasNativeWindowClosed() const override;
+
+ private:
+  // aura::WindowObserver:
+  void OnWindowDestroying(aura::Window* window) override;
+
+  aura::Window* window_;
+
+  DISALLOW_COPY_AND_ASSIGN(NativeWindowTrackerAura);
+};
+
+#endif  // CHROME_BROWSER_UI_AURA_NATIVE_WINDOW_TRACKER_AURA_H_