Add support for CrOS drivefs in SanitizeDownloadTargetPath

Clean up all checks to use IsParent which is simpler to understand
than AppendRelativePath.

Also fixes linux files to allow root mount point as valid downloads dir.

Change-Id: Ie7d688949a69262b35bf999f0fc661a4633356c2
Reviewed-on: https://chromium-review.googlesource.com/c/1286235
Commit-Queue: Joel Hockey <joelhockey@chromium.org>
Reviewed-by: Min Qin <qinmin@chromium.org>
Reviewed-by: Sam McNally <sammc@chromium.org>
Cr-Commit-Position: refs/heads/master@{#600581}
2 files changed