[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.

Originally landed as: https://crrev.com/10156ea4faa45bc0a61817c07659dc4c
Reverted as         : https://crrev.com/c1f63eb16417863e361a5de3265bd0fa

Relanding after removing change to enable a PPAPI browser test on macOS.

Bug: 917842
Bug: 927074
TBR: wfh@chromium.org
Change-Id: I911776ec2c6d30f0ed24761f4f1257a62b3b62d6
Reviewed-on: https://chromium-review.googlesource.com/c/1447122
Reviewed-by: Avi Drissman <avi@chromium.org>
Reviewed-by: Min Qin <qinmin@chromium.org>
Commit-Queue: Avi Drissman <avi@chromium.org>
Auto-Submit: Asanka Herath <asanka@chromium.org>
Cr-Commit-Position: refs/heads/master@{#628430}
10 files changed