Fix crashes in DownloadRequestLimiter when extension popups/bubbles initiate downloads automatically

chrome/browser/download/download_request_limiter_observer.h

 // Copyright (c) 2011 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_DOWNLOAD_DOWNLOAD_REQUEST_LIMITER_OBSERVER_H_
 #define CHROME_BROWSER_DOWNLOAD_DOWNLOAD_REQUEST_LIMITER_OBSERVER_H_
 
+#include "base/callback.h"
 #include "content/public/browser/web_contents_observer.h"
 
 // Watches for user gesture notifications.
 class DownloadRequestLimiterObserver : public content::WebContentsObserver {
  public:
-  explicit DownloadRequestLimiterObserver(content::WebContents* web_contents);
+  typedef base::Callback<void(DownloadRequestLimiterObserver*)>
+    DestroyedCallback;
+
+  DownloadRequestLimiterObserver(
+      content::WebContents* web_contents,
+      const DestroyedCallback& on_destroyed);
   virtual ~DownloadRequestLimiterObserver();
 
   // content::WebContentsObserver overrides.
   virtual void DidGetUserGesture() OVERRIDE;
+  virtual void WebContentsDestroyed(
+      content::WebContents* web_contents) OVERRIDE;
 
  private:
+  DestroyedCallback on_destroyed_;
+
   DISALLOW_COPY_AND_ASSIGN(DownloadRequestLimiterObserver);
 };
 
 #endif  // CHROME_BROWSER_DOWNLOAD_DOWNLOAD_REQUEST_LIMITER_OBSERVER_H_