Only show Last N Pages in the UI when the corresponding tab is visible.

components/offline_pages/core/downloads/download_ui_adapter.h

Index: components/offline_pages/core/downloads/download_ui_adapter.h
diff --git a/components/offline_pages/core/downloads/download_ui_adapter.h b/components/offline_pages/core/downloads/download_ui_adapter.h
index abf413a139e814ef4480f9766b8916a31590c9ec..112356cdfde58648cf4a44b217b6785cdea6f872 100644
--- a/components/offline_pages/core/downloads/download_ui_adapter.h
+++ b/components/offline_pages/core/downloads/download_ui_adapter.h
@@ -113,12 +113,14 @@ class DownloadUIAdapter : public OfflinePageModel::Observer,
   // change.
   void TemporaryHiddenStatusChanged(const ClientId& client_id);
 
+  Delegate* delegate() { return delegate_.get(); }
+
  private:
   enum class State { NOT_LOADED, LOADING_PAGES, LOADING_REQUESTS, LOADED };
 
   struct ItemInfo {
-    explicit ItemInfo(const OfflinePageItem& page);
-    explicit ItemInfo(const SavePageRequest& request);
+    ItemInfo(Delegate* delegate, const OfflinePageItem& page);

[Dmitry Titov, 2017/02/14 23:01:03]

It'd be nice to avoid passing a delegate simply to pull the 'temporary hidden'
bit. Rather can pass a bit.

ItmeInfo is good as a data bag, and it doesn't keep pointers to Adapter nor
Delegate, so it'd be nice to keep it this way.

[dewittj, 2017/02/15 19:49:45]

Done.

+    ItemInfo(Delegate* delegate, const SavePageRequest& request);
     ~ItemInfo();
 
     std::unique_ptr<DownloadUIItem> ui_item;
@@ -158,6 +160,9 @@ class DownloadUIAdapter : public OfflinePageModel::Observer,
   void OnDeletePagesDone(DeletePageResult result);
 
   void AddItemHelper(std::unique_ptr<ItemInfo> item_info);
+  // This function is not re-entrant.  It temporarily sets |deleting_item_|
+  // while it runs, so that functions such as |GetOfflineIdByGuid| will work
+  // during the |ItemDeleted| callback.
   void DeleteItemHelper(const std::string& guid);
 
   // Always valid, this class is a member of the model.
@@ -174,6 +179,8 @@ class DownloadUIAdapter : public OfflinePageModel::Observer,
   // The cache of UI items. The key is DownloadUIItem.guid.
   DownloadUIItems items_;
 
+  std::unique_ptr<ItemInfo> deleting_item_;
+
   // The observers.
   base::ObserverList<Observer> observers_;
   int observers_count_;