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This allows CPanel to disable TLSv1.1/TLSv1.2 for compatibility with
buggy wifi APs.

BUG=605728
TEST=chromeos_unittests
TEST="sideload" an .onc file through chrome://net-internals and verify
     in the RADIUS logs that the TLSVersionMax setting takes effect
TEST=manually set value through shill and check in
     networkingPrivate.getProperties

Change-Id: I1c8447ca5f7c8b0c8ff3c4ce44be01955454d215
Reviewed-on: https://chromium-review.googlesource.com/789792Reviewed-by: Steven Bennetts <stevenjb@chromium.org>
Reviewed-by: Toni Barzic <tbarzic@chromium.org>
Commit-Queue: Kevin Cernekee <cernekee@chromium.org>
Cr-Commit-Position: refs/heads/master@{#519926}