Add Enterprise policy option to control minimum SSL fallback level.

r299567 disabled SSLv3 fallback. This change adds an enterprise policy option
to control this value so that enterprises can reenable the fallback if they
need it.

This option is in contrast to the one added in r299755. That one allows
enterprises to be more aggressive in disabling SSLv3, while this one is
intended for those who need to more slower that Chrome's default.

BUG=418848,419870

Review URL: https://codereview.chromium.org/644913008

(cherry picked from commit 33369a4034528613b79464015cf8a000e3f27abb)

Conflicts:
	components/policy/resources/policy_templates.json

Cr-Commit-Position: refs/branch-heads/2171@{#222}
Cr-Branched-From: 267aeeb8d85c8503a7fd12bd14654b8ea78d3974-refs/heads/master@{#297060}
5 files changed