This CL sets up a download internal directory which can be used by clients
such as background fetch to store downloaded files that are not publicly
visible. Callers of DownloadManager::DownloadUrl can specify a download
path which in conjunction with transient flag will set up a pre-set path
for the downloaded file.

BUG=710669

Review-Url: https://codereview.chromium.org/2811173002
Cr-Commit-Position: refs/heads/master@{#465066}