Allow for kStateDisconnect to be received from Shill

CL:1591595 removed kStateDisconnect usage/support from NetworkState
because Shill had not been using that Service state for years. $bug
led to the conclusion that Shill should have a disconnecting state so
that the potentially non-negligible time between beginning a
disconnection and completing the disconnection will be clear (both
internally to Shill and for Shill D-Bus clients), rather than having
the Service Disconnect D-Bus method block until completion. This CL is
done in preparation of re-introducing the disconnect state to Shill,
such that future Shill CLs will cause Chrome to trigger DCHECKs.

Note that many of the changes in CL:1591595 are necessary in order to
avoid a future scenario where different usages of the kStateDisconnect
state (received from Shill vs Chrome-internal usages) have different
semantics.

Bug: 989361
Change-Id: I3d2ef136d531a39d5e1f89da77321d1866ac28aa
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1819630
Commit-Queue: Alex Khouderchah <akhouderchah@chromium.org>
Reviewed-by: Kyle Horimoto <khorimoto@chromium.org>
Cr-Commit-Position: refs/heads/master@{#699378}

chromeos/network/network_state.cc

@@ -411,6 +411,7 @@ std::string NetworkState::connection_state() const {
          connection_state_ == shill::kStateOffline ||
          connection_state_ == shill::kStateOnline ||
          connection_state_ == shill::kStateFailure ||
+         connection_state_ == shill::kStateDisconnect ||
          // TODO(https://crbug.com/552190): Remove kStateActivationFailure from
          // this list when occurrences in chromium code have been eliminated.
          connection_state_ == shill::kStateActivationFailure ||