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

chrome/browser/ui/cocoa/native_window_tracker_cocoa.h

Index: chrome/browser/ui/cocoa/native_window_tracker_cocoa.h
diff --git a/chrome/browser/ui/cocoa/native_window_tracker_cocoa.h b/chrome/browser/ui/cocoa/native_window_tracker_cocoa.h
new file mode 100644
index 0000000000000000000000000000000000000000..7704adc5a0c0f044605d59b412c5a23994ec769a
--- /dev/null
+++ b/chrome/browser/ui/cocoa/native_window_tracker_cocoa.h
@@ -0,0 +1,27 @@
+// 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_COCOA_NATIVE_WINDOW_TRACKER_COCOA_H_
+#define CHROME_BROWSER_UI_COCOA_NATIVE_WINDOW_TRACKER_COCOA_H_
+
+#include "base/mac/scoped_nsobject.h"
+#include "chrome/browser/ui/native_window_tracker.h"
+
+@class BridgedNativeWindowTracker;
+
+class NativeWindowTrackerCocoa : public NativeWindowTracker {
+ public:
+  explicit NativeWindowTrackerCocoa(gfx::NativeWindow window);
+  ~NativeWindowTrackerCocoa() override;
+
+  // NativeWindowTracker:
+  bool WasNativeWindowClosed() const override;
+
+ private:
+  base::scoped_nsobject<BridgedNativeWindowTracker> bridge_;
+
+  DISALLOW_COPY_AND_ASSIGN(NativeWindowTrackerCocoa);
+};
+
+#endif  // CHROME_BROWSER_UI_COCOA_NATIVE_WINDOW_TRACKER_COCOA_H_