[Downloads] Rework how hashes are calculated for download files.

chrome/browser/download/download_history.cc

 // Copyright (c) 2012 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.
 
 // DownloadHistory manages persisting DownloadItems to the history service by
 // observing a single DownloadManager and all its DownloadItems using an
 // AllDownloadItemNotifier.
 //
 // DownloadHistory decides whether and when to add items to, remove items from,
 // and update items in the database. DownloadHistory uses DownloadHistoryData to
@@ skipping 264 matching lines @@
         it->url_chain,
         it->referrer_url,
         it->mime_type,
         it->original_mime_type,
         it->start_time,
         it->end_time,
         it->etag,
         it->last_modified,
         it->received_bytes,
         it->total_bytes,
+        std::string(),  // TODO(asanka): Need to persist and restore hash of
+                        // partial file for an interrupted download. No need to
+                        // store hash for a completed file.
         history::ToContentDownloadState(it->state),
         history::ToContentDownloadDangerType(it->danger_type),
         history::ToContentDownloadInterruptReason(it->interrupt_reason),
         it->opened);
 #if defined(ENABLE_EXTENSIONS)
     if (!it->by_ext_id.empty() && !it->by_ext_name.empty()) {
       new extensions::DownloadedByExtension(
           item, it->by_ext_id, it->by_ext_name);
       item->UpdateObservers();
     }
@@ skipping 170 matching lines @@
   removing_ids_.insert(download_id);
 }
 
 void DownloadHistory::RemoveDownloadsBatch() {
   DCHECK_CURRENTLY_ON(content::BrowserThread::UI);
   IdSet remove_ids;
   removing_ids_.swap(remove_ids);
   history_->RemoveDownloads(remove_ids);
   FOR_EACH_OBSERVER(Observer, observers_, OnDownloadsRemoved(remove_ids));
 }