Add new UMA stats to get a handle on Downloads UI Usage

content/browser/download/download_manager.h

 // Copyright (c) 2011 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.
 //
 // The DownloadManager object manages the process of downloading, including
 // updates to the history system and providing the information for displaying
 // the downloads view in the Destinations tab. There is one DownloadManager per
 // active browser context in Chrome.
 //
 // Download observers:
@@ skipping 245 matching lines @@
   virtual int64 GetTotalDownloadBytes();
 
   // Called by the delegate after the save as dialog is closed.
   void FileSelected(const FilePath& path, void* params);
   void FileSelectionCanceled(void* params);
 
   // Called by the delegate if it delayed the download in
   // DownloadManagerDelegate::ShouldStartDownload and now is ready.
   void RestartDownload(int32 download_id);
 
+  // Mark the download opened in the persistent store.
+  void MarkDownloadOpened(DownloadItem* download);
+
   // Checks whether downloaded files still exist. Updates state of downloads
   // that refer to removed files. The check runs in the background and may
   // finish asynchronously after this method returns.
   void CheckForHistoryFilesRemoval();
 
   // Checks whether a downloaded file still exists and updates the file's state
   // if the file is already removed. The check runs in the background and may
   // finish asynchronously after this method returns.
   void CheckForFileRemoval(DownloadItem* download_item);
 
@@ skipping 151 matching lines @@
   DownloadManagerDelegate* delegate_;
 
   // TODO(rdsmith): Remove when http://crbug.com/84508 is fixed.
   // For debugging only.
   int64 largest_db_handle_in_history_;
 
   DISALLOW_COPY_AND_ASSIGN(DownloadManager);
 };
 
 #endif  // CONTENT_BROWSER_DOWNLOAD_DOWNLOAD_MANAGER_H_