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The new class AlwaysOnVpnManager listens to changes in the
Always-on VPN preferences, and forwards any changes to shill
to configure the network correctly.

BUG=b:72861260
BUG=873123
TEST=unit tests

Change-Id: I287f52c7326645085e98d9bd59fd94fd9a692f5e
Reviewed-on: https://chromium-review.googlesource.com/1052508
Commit-Queue: Philipp Weiß <phweiss@chromium.org>
Reviewed-by: Steven Bennetts <stevenjb@chromium.org>
Reviewed-by: Jochen Eisinger <jochen@chromium.org>
Reviewed-by: Ben Chan <benchan@chromium.org>
Reviewed-by: Luis Hector Chavez <lhchavez@chromium.org>
Cr-Commit-Position: refs/heads/master@{#582149}