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- Uses the the Archive API to iterative over the files
  contained inside the downloaded .rar file.
- Identify if the .rar file contains any executables.
- Identify if the .rar file contains any archives.

This information isn't currently used to populate the CSBRR ping
sent to Safe Browsing. That's work for a follow-up CL.

All this is behind the flag: InspectDownloadedRarFiles

Note: This CL sends a fd/handle to the unrar library since code
running inside the sandbox can't open/create files.

Change-Id: Ie5d30040f51f59854d4a041652d171d704f4df78
Bug: 750327
Reviewed-on: https://chromium-review.googlesource.com/947664
Commit-Queue: Varun Khaneja <vakh@chromium.org>
Reviewed-by: Nathan Parker <nparker@chromium.org>
Reviewed-by: Will Harris <wfh@chromium.org>
Reviewed-by: Ken Rockot <rockot@chromium.org>
Reviewed-by: Luke Z <lpz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#563016}