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This adds the DownloadConnectionSecurity of the download and a boolean
for whether the final download URL is a different host than the
originating URL to the existing Download.Started UKM.

This also refactors out the logic for checking the
DownloadConnectionSecurity into a new public function
(CheckDownloadConnectionSecurity) in download_stats.h and exposes the
underlying DownloadConnectionSecurity enum.

This will help us identify sites that have a lot of downloads over
HTTP (or go over HTTP in their redirect chain) for outreach efforts.

Bug: 839986
Change-Id: I9a1bb78b8c62547e322ec1b85b5acfaa7acc813a
Reviewed-on: https://chromium-review.googlesource.com/1195854
Commit-Queue: Christopher Thompson <cthomp@chromium.org>
Reviewed-by: Robert Kaplow (slow) <rkaplow@chromium.org>
Reviewed-by: Min Qin <qinmin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#587737}