[Quarantine] Attach source metadata to files only on Windows and Mac.

This change stops attaching the origin and referrer URL metadata to
downloaded files on Linux. This information was previously set via the
user.xdg.origin.url and user.xdg.referrer.url xattrs. Attaching this
metadata doesn't provide any security guarantees on Linux, and is a
privacy risk.

R=wfh@chromium.org

Bug: 917842
Change-Id: If0586d9b1c340f51ebc480f3c2281cc17e478c5e
Reviewed-on: https://chromium-review.googlesource.com/c/1407441
Commit-Queue: Asanka Herath <asanka@chromium.org>
Reviewed-by: Avi Drissman <avi@chromium.org>
Reviewed-by: Will Harris <wfh@chromium.org>
Reviewed-by: Min Qin <qinmin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#627488}
10 files changed