Design doc for usbguard on Chrome OS: https://docs.google.com/document/d/1wstNeVI62Eh6Rm6zqszHpiZphbxAoVGybsvbKGpYjXQ/edit?usp=sharing

The following revision refers to this bug:
  https://chromium.googlesource.com/chromiumos/platform2/+/05707d89271fad409611dced902d3faf21c5faf2

commit 05707d89271fad409611dced902d3faf21c5faf2
Author: Allen Webb <allenwebb@google.com>
Date: Tue Oct 23 20:01:51 2018

session_manager: Add IsScreenLocked() dbus method.

This adds the IsScreenLocked() D-Bus method along with test coverage. A
single boolean is returned indicated true when the session is locked.

This will be used to determine which USB devices were added while at
the lock screen to keep them from being whitelisted until after unlock.

BUG=chromium:896677
TEST=FEATURES=test emerge-${BOARD} chromeos-login

Change-Id: Ie50491b60e0255d09cf7da09a50a429af5f403bd
Signed-off-by: Allen Webb <allenwebb@google.com>
Reviewed-on: https://chromium-review.googlesource.com/1287710
Commit-Ready: ChromeOS CL Exonerator Bot <chromiumos-cl-exonerator@appspot.gserviceaccount.com>
Reviewed-by: Mattias Nissler <mnissler@chromium.org>
Reviewed-by: Dan Erat <derat@chromium.org>

[modify] https://crrev.com/05707d89271fad409611dced902d3faf21c5faf2/login_manager/session_manager_impl.cc
[modify] https://crrev.com/05707d89271fad409611dced902d3faf21c5faf2/login_manager/dbus_bindings/org.chromium.SessionManagerInterface.xml
[modify] https://crrev.com/05707d89271fad409611dced902d3faf21c5faf2/system_api/dbus/login_manager/dbus-constants.h
[modify] https://crrev.com/05707d89271fad409611dced902d3faf21c5faf2/login_manager/session_manager_impl_test.cc
[modify] https://crrev.com/05707d89271fad409611dced902d3faf21c5faf2/login_manager/session_manager_impl.h

Issue 921058 has been merged into this issue.