[Offline pages] Fixing crash happening when 2 pages get saved about the same time

components/offline_pages/downloads/download_ui_adapter.cc

 // Copyright 2016 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 "components/offline_pages/downloads/download_ui_adapter.h"
 
 #include "base/bind.h"
 #include "base/guid.h"
 #include "base/logging.h"
 #include "base/memory/ptr_util.h"
@@ skipping 180 matching lines @@
   std::vector<std::string> added_guids;
   for (const auto& page : pages) {
     if (!IsVisibleInUI(page.client_id))  // Item should be filtered out.
       continue;
     const std::string& guid = page.client_id.id;
     if (items_.find(guid) != items_.end())  // Item already exists.
       continue;
     items_[guid] = base::MakeUnique<ItemInfo>(page);
     added_guids.push_back(guid);
   }
-  // Only one added page
-  CHECK(added_guids.size() <= 1);
   for (auto& guid : added_guids) {
     const DownloadUIItem& item = *(items_.find(guid)->second->ui_item.get());
     FOR_EACH_OBSERVER(Observer, observers_, ItemAdded(item));
   }
 }
 
 void DownloadUIAdapter::OnDeletePagesDone(DeletePageResult result) {
   // TODO(dimich): Consider adding UMA to record user actions.
 }
 
 // static
 bool DownloadUIAdapter::IsVisibleInUI(const ClientId& client_id) {
   const std::string& name_space = client_id.name_space;
   return (name_space == kAsyncNamespace || name_space == kDownloadNamespace) &&
          base::IsValidGUID(client_id.id);
 }
 
 }  // namespace offline_pages