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

content/browser/download/download_manager_delegate.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.
 
 #ifndef CONTENT_BROWSER_DOWNLOAD_DOWNLOAD_MANAGER_DELEGATE_H_
 #define CONTENT_BROWSER_DOWNLOAD_DOWNLOAD_MANAGER_DELEGATE_H_
 #pragma once
 
 #include "base/basictypes.h"
 #include "base/memory/weak_ptr.h"
@@ skipping 56 matching lines @@
                                    const std::string& hash) = 0;
 
   // Notifies the delegate that a new download item is created. The
   // DownloadManager waits for the delegate to add information about this
   // download to its persistent store. When the delegate is done, it calls
   // DownloadManager::OnDownloadItemAddedToPersistentStore.
   virtual void AddItemToPersistentStore(DownloadItem* item) = 0;
 
   // Notifies the delegate that information about the given download has change,
   // so that it can update its persistent store.
+  // Does not update |url|, |start_time|, |total_bytes|; uses |db_handle| only
+  // to select the row in the database table to update.
   virtual void UpdateItemInPersistentStore(DownloadItem* item) = 0;
 
   // Notifies the delegate that path for the download item has changed, so that
   // it can update its persistent store.
   virtual void UpdatePathForItemInPersistentStore(
       DownloadItem* item,
       const FilePath& new_path) = 0;
 
   // Notifies the delegate that it should remove the download item from its
   // persistent store.
@@ skipping 18 matching lines @@
   // Informs the delegate that the progress of downloads has changed.
   virtual void DownloadProgressUpdated() = 0;
 
  protected:
   DownloadManagerDelegate() {}
 
   DISALLOW_COPY_AND_ASSIGN(DownloadManagerDelegate);
 };
 
 #endif  // CONTENT_BROWSER_DOWNLOAD_DOWNLOAD_MANAGER_DELEGATE_H_