downloads: add the ability to undo download removal.

chrome/browser/download/all_download_item_notifier.h

Index: chrome/browser/download/all_download_item_notifier.h
diff --git a/chrome/browser/download/all_download_item_notifier.h b/chrome/browser/download/all_download_item_notifier.h
index 86d4d68753058975b008c7a20dadc6e033edbb28..03334f437fe4435ea1b15c7018c30964e143e06c 100644
--- a/chrome/browser/download/all_download_item_notifier.h
+++ b/chrome/browser/download/all_download_item_notifier.h
@@ -51,6 +51,8 @@ class AllDownloadItemNotifier : public content::DownloadManager::Observer,
         content::DownloadManager* manager, content::DownloadItem* item) {}
     virtual void OnDownloadRemoved(
         content::DownloadManager* manager, content::DownloadItem* item) {}
+    virtual void OnDownloadRevived(
+        content::DownloadManager* manager, content::DownloadItem* item) {}
 
    private:
     DISALLOW_COPY_AND_ASSIGN(Observer);
@@ -69,6 +71,8 @@ class AllDownloadItemNotifier : public content::DownloadManager::Observer,
   void ManagerGoingDown(content::DownloadManager* manager) override;
   void OnDownloadCreated(content::DownloadManager* manager,
                          content::DownloadItem* item) override;
+  void OnDownloadRevived(content::DownloadManager* manager,
+                         content::DownloadItem* item) override;
 
   // content::DownloadItem::Observer
   void OnDownloadUpdated(content::DownloadItem* item) override;