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This migrates chromeos::UserSessionManager from NetworkChangeNotifier to
NetworkConnectionTracker, which works with the network service enabled.

Bug: 887047
Change-Id: I814311aa08ab2dafa7bf9b4ac59b3dab0e0a984e
Reviewed-on: https://chromium-review.googlesource.com/c/1237553
Commit-Queue: Robbie McElrath <rmcelrath@chromium.org>
Reviewed-by: proberge <proberge@chromium.org>
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#632382}