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

chrome/browser/ui/cocoa/native_window_tracker_cocoa.mm

Index: chrome/browser/ui/cocoa/native_window_tracker_cocoa.mm
diff --git a/chrome/browser/ui/cocoa/native_window_tracker_cocoa.mm b/chrome/browser/ui/cocoa/native_window_tracker_cocoa.mm
new file mode 100644
index 0000000000000000000000000000000000000000..14afb8d9550177e1b29fd471ccc306c8085f0479
--- /dev/null
+++ b/chrome/browser/ui/cocoa/native_window_tracker_cocoa.mm
@@ -0,0 +1,66 @@
+// 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/cocoa/native_window_tracker_cocoa.h"
+
+#import <AppKit/AppKit.h>
+
+@interface BridgedNativeWindowTracker : NSObject {
+ @private
+  NSWindow* window_;
+}
+
+- (id)initWithNSWindow:(NSWindow*)window;
+- (bool)wasNSWindowClosed;
+- (void)onWindowWillClose:(NSNotification*)notification;
+
+@end
+
+@implementation BridgedNativeWindowTracker
+
+- (id)initWithNSWindow:(NSWindow*)window {
+  window_ = window;
+  NSNotificationCenter* center = [NSNotificationCenter defaultCenter];
+  [center addObserver:self
+             selector:@selector(onWindowWillClose:)
+                 name:NSWindowWillCloseNotification
+               object:window_];
+  return self;
+}
+
+- (void)dealloc {
+  [[NSNotificationCenter defaultCenter] removeObserver:self];
+  [super dealloc];
+}
+
+- (bool)wasNSWindowClosed {
+  return window_ == nil;
+}
+
+- (void)onWindowWillClose:(NSNotification*)notification {
+  [[NSNotificationCenter defaultCenter]
+      removeObserver:self
+                name:NSWindowWillCloseNotification
+              object:window_];
+  window_ = nil;
+}
+
+@end
+
+NativeWindowTrackerCocoa::NativeWindowTrackerCocoa(gfx::NativeWindow window) {
+  bridge_.reset([[BridgedNativeWindowTracker alloc] initWithNSWindow:window]);
+}
+
+NativeWindowTrackerCocoa::~NativeWindowTrackerCocoa() {
+}
+
+bool NativeWindowTrackerCocoa::WasNativeWindowClosed() const {
+  return [bridge_ wasNSWindowClosed];
+}
+
+// static
+scoped_ptr<NativeWindowTracker> NativeWindowTracker::Create(
+    gfx::NativeWindow window) {
+  return scoped_ptr<NativeWindowTracker>(new NativeWindowTrackerCocoa(window));
+}