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

chrome/browser/download/download_request_limiter_observer.cc

 // 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.
 
 #include "chrome/browser/download/download_request_limiter_observer.h"
 
 #include "chrome/browser/browser_process.h"
 #include "chrome/browser/download/download_request_limiter.h"
 
 using content::WebContents;
 
 DownloadRequestLimiterObserver::DownloadRequestLimiterObserver(
-  WebContents* web_contents)
-    : content::WebContentsObserver(web_contents) {
+  WebContents* web_contents,
+  const DownloadRequestLimiterObserver::DestroyedCallback& on_destroyed)
+    : content::WebContentsObserver(web_contents),
+      on_destroyed_(on_destroyed) {
 }
 
 DownloadRequestLimiterObserver::~DownloadRequestLimiterObserver() {
 }
 
 void DownloadRequestLimiterObserver::DidGetUserGesture() {
   if (!g_browser_process->download_request_limiter())
     return;  // NULL in unitests.
   g_browser_process->download_request_limiter()->OnUserGesture(web_contents());
 }
+
+void DownloadRequestLimiterObserver::WebContentsDestroyed(
+    WebContents* contents) {
+  if (!on_destroyed_.is_null())
+    on_destroyed_.Run(this);
+  delete this;
+}