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This method is intended to be used by the smart-card extension to monitor login
status.

* cros-specific usersPrivate.getLoginStatus returns a dictionary with
data members isLoggedIn and isScreenLocked.
* tests for logged-in user, and screenlock cases.

Bug: 777356
Test: browser_tests --gtest_filter=UsersPrivateApiLo\*
Cq-Include-Trybots: luci.chromium.try:closure_compilation
Change-Id: Ib0ba29d3c715fb0ce162ba234ca9be657d40582c
Reviewed-on: https://chromium-review.googlesource.com/1094203
Commit-Queue: Achuith Bhandarkar <achuith@chromium.org>
Reviewed-by: Steven Bennetts <stevenjb@chromium.org>
Reviewed-by: Achuith Bhandarkar <achuith@chromium.org>
Reviewed-by: Reilly Grant <reillyg@chromium.org>
Reviewed-by: Maksim Ivanov <emaxx@chromium.org>
Cr-Commit-Position: refs/heads/master@{#576235}