HistoryBackend: Fix a crash in BroadcastNotification.

chrome/browser/history/history_backend.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/history/history_backend.h"
 
 #include <list>
 #include <map>
 #include <set>
 #include <vector>
@@ skipping 2054 matching lines @@
   db_task_requests_.push_back(request.get());
   if (!task_scheduled) {
     // No other tasks are scheduled. Process request now.
     ProcessDBTaskImpl();
   }
 }
 
 void HistoryBackend::BroadcastNotifications(
     int type,
     HistoryDetails* details_deleted) {
-  DCHECK(delegate_.get());
-  delegate_->BroadcastNotifications(type, details_deleted);
+  // |delegate_| may be NULL if |this| is in the process of closing (closed by
+  // HistoryService -> HistroyBackend::Closing().
+  if (delegate_.get())
+    delegate_->BroadcastNotifications(type, details_deleted);
 }
 
 // Deleting --------------------------------------------------------------------
 
 void HistoryBackend::DeleteAllHistory() {
   // Our approach to deleting all history is:
   //  1. Copy the bookmarks and their dependencies to new tables with temporary
   //     names.
   //  2. Delete the original tables. Since tables can not share pages, we know
   //     that any data we don't want to keep is now in an unused page.
@@ skipping 212 matching lines @@
         break;
       }
     }
   }
   UMA_HISTOGRAM_TIMES("History.GetFavIconFromDB",  // historical name
                       TimeTicks::Now() - beginning_time);
   return success;
 }
 
 }  // namespace history