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..a8562be7bfc6bf156ff51e429c9d3a2e8d3cc4ea 100644
--- a/components/offline_pages/core/downloads/download_ui_adapter.h
+++ b/components/offline_pages/core/downloads/download_ui_adapter.h
@@ -1,4 +1,4 @@
-// Copyright 2016 The Chromium Authors. All rights reserved.
+// Copyright 2017 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.
 
@@ -48,6 +48,10 @@ class DownloadUIAdapter : public OfflinePageModel::Observer,
     // of temporary visibility changes, the Delegate is supposed to call
     // DownloadUIAdapter::TemporarilyHiddenStatusChanged().
     virtual bool IsTemporarilyHiddenInUI(const ClientId& client_id) = 0;
+
+    // Delegates need a reference to the UI adapter in order to notify it about
+    // visibility changes.
+    virtual void SetUIAdapter(DownloadUIAdapter* ui_adapter) = 0;
   };
 
   // Observer, normally implemented by UI or a Bridge.
@@ -113,12 +117,14 @@ 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(const OfflinePageItem& page, bool temporarily_hidden);
+    ItemInfo(const SavePageRequest& request, bool temporarily_hidden);
     ~ItemInfo();
 
     std::unique_ptr<DownloadUIItem> ui_item;
@@ -158,6 +164,9 @@ 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 +183,8 @@ 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_;