Although we want to prevent dfgs from untar'ing files to a parent
or sibling of its target directory, normal files that just happen
to have ".." in their name (i.e. not preceding a path separator) are
okay.

R=hinoka

Bug: 807286
Change-Id: Ibdc2c3615c4778ef66abceb532a4f671fbdab8ef
Reviewed-on: https://chromium-review.googlesource.com/912430Reviewed-by: Ryan Tseng <hinoka@chromium.org>
Commit-Queue: Aaron Gable <agable@chromium.org>