Fix a crash in DownloadUIAddapter that happens when Java bridge is created/removed quickly while

components/offline_pages/downloads/download_ui_adapter.h

 // 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.
 
 #ifndef COMPONENTS_OFFLINE_PAGE_DOWNLOADS_DOWNLOAD_UI_ADAPTER_H_
 #define COMPONENTS_OFFLINE_PAGE_DOWNLOADS_DOWNLOAD_UI_ADAPTER_H_
 
 #include <map>
 #include <memory>
 #include <string>
@@ skipping 63 matching lines @@
   void DeleteItem(const std::string& guid);
   GURL GetOfflineUrlByGuid(const std::string& guid) const;
 
   // OfflinePageModel::Observer
   void OfflinePageModelLoaded(OfflinePageModel* model) override;
   void OfflinePageModelChanged(OfflinePageModel* model) override;
   void OfflinePageDeleted(int64_t offline_id,
                           const ClientId& client_id) override;
 
  private:
+  enum class State {
+    NOT_LOADED,
+    LOADING,
+    LOADED
+  };
+
   struct ItemInfo {
     explicit ItemInfo(const OfflinePageItem& page);
     ~ItemInfo();
 
     std::unique_ptr<DownloadUIItem> ui_item;
     // Additional cached data, not exposed to UI through DownloadUIItem.
     int64_t offline_id;
     GURL offline_url;
 
    private:
     DISALLOW_COPY_AND_ASSIGN(ItemInfo);
   };
 
   typedef std::map<std::string, std::unique_ptr<ItemInfo>> DownloadUIItems;
 
   void LoadCache();
   void ClearCache();
 
   // Task callbacks.
   void OnOfflinePagesLoaded(const MultipleOfflinePageItemResult& pages);
   void NotifyItemsLoaded(Observer* observer);
   void OnOfflinePagesChanged(const MultipleOfflinePageItemResult& pages);
   void OnDeletePagesDone(DeletePageResult result);
 
   bool IsVisibleInUI(const ClientId& page);
 
   // Always valid, this class is a member of the model.
   OfflinePageModel* model_;
 
-  bool is_loaded_;
+  State state_;
 
   // The cache of UI items. The key is DownloadUIItem.guid.
   DownloadUIItems items_;
 
   // The observers.
   base::ObserverList<Observer> observers_;
+  int observers_count_;
 
   base::WeakPtrFactory<DownloadUIAdapter> weak_ptr_factory_;
 
   DISALLOW_COPY_AND_ASSIGN(DownloadUIAdapter);
 };
 
 }  // namespace offline_pages
 
 #endif  // COMPONENTS_OFFLINE_PAGE_DOWNLOADS_DOWNLOAD_UI_ADAPTER_H_