add a download slices table into history download db

components/history/core/browser/download_constants.h

 // Copyright 2015 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 COMPONENTS_HISTORY_CORE_BROWSER_DOWNLOAD_CONSTANTS_H_
 #define COMPONENTS_HISTORY_CORE_BROWSER_DOWNLOAD_CONSTANTS_H_
 
+#include "base/feature_list.h"
 #include "components/history/core/browser/download_types.h"
 
 namespace history {
 
 // DownloadState represents the state of a DownloadRow saved into the
 // DownloadDatabase. The values must not be changed as they are saved
 // to disk in the database.
 enum class DownloadState {
   INVALID = -1,
   IN_PROGRESS = 0,
@@ skipping 17 matching lines @@
   USER_VALIDATED = 6,
   DANGEROUS_HOST = 7,
   POTENTIALLY_UNWANTED = 8,
 };
 
 // DownloadId represents the id of a DownloadRow into the DownloadDatabase.
 // The value is controlled by the embedder except for the reserved id
 // kInvalidDownloadId.
 extern const DownloadId kInvalidDownloadId;
 
+// This feature allows parallel url requests to be issued for a single download
+// item.
+extern const base::Feature kParallelDownloading;
 }  // namespace
 
 #endif  // COMPONENTS_HISTORY_CORE_BROWSER_DOWNLOAD_CONSTANTS_H_