GitLab

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}