[CrOS SplitSettings] Fork OS download settings into new files

Remove all browser download settings.
Hide OS specific download settings in Browser settings.

Change-Id: Id098ee72dd67b52fd25896f06b98318e2ba6771f
Bug: 950007
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1616999
Commit-Queue: Regan Hsu <hsuregan@chromium.org>
Reviewed-by: Demetrios Papadopoulos <dpapad@chromium.org>
Cr-Commit-Position: refs/heads/master@{#661956}
11 files changed