safe browsing: allow disabling dependency on unrar

The license of unrar is considered incompatible with DFSG / OSI OSD
(rule 6), and FSF FSD (freedom 0). Some distros follow that strictly
and maintain patches to remove it independently. This CL upstreams it
as a simple build option.

Change-Id: Ib23e650c352c6c7c2dd44f0e0a3b8f27515c2724
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5644830
Reviewed-by: Xinghui Lu <xinghuilu@chromium.org>
Reviewed-by: Daniel Rubery <drubery@chromium.org>
Commit-Queue: Daniel Rubery <drubery@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1322831}