add a download slices table into history download db

components/history/core/browser/download_constants.h

Index: components/history/core/browser/download_constants.h
diff --git a/components/history/core/browser/download_constants.h b/components/history/core/browser/download_constants.h
index ea21725fafa40443bf471be97ea1507ea226bda4..8d0850fb887d07f1108549abce7025fd29a293f8 100644
--- a/components/history/core/browser/download_constants.h
+++ b/components/history/core/browser/download_constants.h
@@ -5,6 +5,7 @@
 #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 {
@@ -42,6 +43,9 @@ enum class DownloadDangerType {
 // 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_