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To prevent attempting to extract other volumes within the sandbox, this
CL puts unrar in NOVOLUME mode. I also added a quick test to ensure we
still detect the presence of files in NOVOLUME mode, so this should be
safe.

Note: The generation of multipart rar files seems a little finicky. The
following two commands worked for me.

dd if=/dev/urandom of=random.exe bs=1k count=1
rar a -v1b multipart.rar random.exe

Bug: 917679, 909778
Change-Id: I472b0e959d7538dc9136597ada631707b044e765
Reviewed-on: https://chromium-review.googlesource.com/c/1393803Reviewed-by: Jay Civelli <jcivelli@chromium.org>
Reviewed-by: Varun Khaneja <vakh@chromium.org>
Commit-Queue: Daniel Rubery <drubery@chromium.org>
Cr-Commit-Position: refs/heads/master@{#620026}