Don't update history db for incompleted download if DownloadDB is enabled

if DownloadDB is enabled, there is no need to update incomplete download
in history db.
The DownloadDB should contain all information.

BUG=863602

Change-Id: Idf20bbb91e8e5156aa21354695b0781493c723ef
Reviewed-on: https://chromium-review.googlesource.com/1137235
Commit-Queue: Min Qin <qinmin@chromium.org>
Reviewed-by: Xing Liu <xingliu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#575873}
2 files changed