The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/5a0d4181b538c577b034233bde90f4aa1a446aa2

commit 5a0d4181b538c577b034233bde90f4aa1a446aa2
Author: Megumi Hattori <megumihattori@google.com>
Date: Tue Mar 06 11:09:50 2018

Show Message Center on lock screen behind flag.

This is a test bed for Lock Screen Notification
(go/notification-on-lock-screen).

This CL adds a new flag enable-lock-screen-notification. Behind this
flag, Message Center with notifications for the selected user is shown
on the lock screen.

Compared with Message Center on the non-locked screen, Message Center
on the lock screen does not have a settings button, which means it does
not have a separator between the settings button and a quiet mode button
either.

BUG= 816353 
TEST=ash_unittests

Change-Id: I27094a8a2c3c8b5f3b48f3ae5c8dccce4bfe78ab
Reviewed-on: https://chromium-review.googlesource.com/939223
Reviewed-by: Tetsui Ohkubo <tetsui@chromium.org>
Reviewed-by: James Cook <jamescook@chromium.org>
Commit-Queue: Megumi Hattori <megumihattori@google.com>
Cr-Commit-Position: refs/heads/master@{#541067}
[modify] https://crrev.com/5a0d4181b538c577b034233bde90f4aa1a446aa2/ash/message_center/message_center_button_bar.cc
[modify] https://crrev.com/5a0d4181b538c577b034233bde90f4aa1a446aa2/ash/message_center/message_center_button_bar.h
[modify] https://crrev.com/5a0d4181b538c577b034233bde90f4aa1a446aa2/ash/message_center/message_center_view.cc
[modify] https://crrev.com/5a0d4181b538c577b034233bde90f4aa1a446aa2/ash/message_center/message_center_view_unittest.cc
[modify] https://crrev.com/5a0d4181b538c577b034233bde90f4aa1a446aa2/ash/public/cpp/ash_features.cc
[modify] https://crrev.com/5a0d4181b538c577b034233bde90f4aa1a446aa2/ash/public/cpp/ash_features.h
[modify] https://crrev.com/5a0d4181b538c577b034233bde90f4aa1a446aa2/ash/system/web_notification/session_state_notification_blocker.cc
[modify] https://crrev.com/5a0d4181b538c577b034233bde90f4aa1a446aa2/chrome/browser/about_flags.cc
[modify] https://crrev.com/5a0d4181b538c577b034233bde90f4aa1a446aa2/chrome/browser/flag_descriptions.cc
[modify] https://crrev.com/5a0d4181b538c577b034233bde90f4aa1a446aa2/chrome/browser/flag_descriptions.h
[modify] https://crrev.com/5a0d4181b538c577b034233bde90f4aa1a446aa2/tools/metrics/histograms/enums.xml