Added flag for new download backend

This CL adds :
1 - An flag for using DownloadOfflineContentProvider in chrome://flags
2 - Handles empty mime types correctly

Bug: 939958
Change-Id: I73b53ad614b014f1d47a388a513577f343577c93
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1512533
Reviewed-by: Min Qin <qinmin@chromium.org>
Commit-Queue: Shakti Sahu <shaktisahu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#640052}
8 files changed