What's the priority on this one? I'm currently focusing on getting a mus_browser_tests built. So I won't have time for a bit.

Not high priority. My assumption is that it is blocked on having a fully-functional pref service that ash can use. Is that right?

Yeah, right now the pref service is only on in mash, or with an extra flag. There are plans to enable it by default at the end of April.

If this isn't high priority I'll grab it, and make the switch once both classic ash and mash can make use of it

Yeah, it would be best if both classic ash and mash worked the same way on this one. Thanks for taking it.

Actually, I'll do this. It's more complex than just monitoring for a pref, so it will fit in well to the existing system tray mojoms I've worked on.

I also requires setting up a device management server in order to manually test it, and I've just figured out how to do that.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/c934896452fd0d3e518d1ffe77f68e0a25e66b34

commit c934896452fd0d3e518d1ffe77f68e0a25e66b34
Author: jamescook <jamescook@chromium.org>
Date: Wed Jun 07 15:10:35 2017

chromeos: Convert system tray session length limit to mojo

Chrome and ash run in different processes under mustash so we need to
eliminate ash-to-chrome delegate methods. This eliminates two methods on
ash::SystemTrayDelegate.

Use the SessionController mojom interface to send the session length limit
to ash from Chrome's local state preferences. (The preferences mojo service
does not yet support local state prefs.)

BUG= 712759 
TEST=added to ash_unittests, chrome unit_tests, manually set up a managed
device using go/dmstug, set the pref via policy and verify that the menu
item and notification appear as expected

Review-Url: https://codereview.chromium.org/2923083002
Cr-Commit-Position: refs/heads/master@{#477646}

[modify] https://crrev.com/c934896452fd0d3e518d1ffe77f68e0a25e66b34/ash/BUILD.gn
[modify] https://crrev.com/c934896452fd0d3e518d1ffe77f68e0a25e66b34/ash/public/interfaces/session_controller.mojom
[modify] https://crrev.com/c934896452fd0d3e518d1ffe77f68e0a25e66b34/ash/session/session_controller.cc
[modify] https://crrev.com/c934896452fd0d3e518d1ffe77f68e0a25e66b34/ash/session/session_controller.h
[modify] https://crrev.com/c934896452fd0d3e518d1ffe77f68e0a25e66b34/ash/session/session_observer.h
[delete] https://crrev.com/4808ce31616eb64d5526fb8fa9ef8b8f6d992df3/ash/system/session/session_length_limit_observer.h
[modify] https://crrev.com/c934896452fd0d3e518d1ffe77f68e0a25e66b34/ash/system/session/tray_session_length_limit.cc
[modify] https://crrev.com/c934896452fd0d3e518d1ffe77f68e0a25e66b34/ash/system/session/tray_session_length_limit.h
[modify] https://crrev.com/c934896452fd0d3e518d1ffe77f68e0a25e66b34/ash/system/session/tray_session_length_limit_unittest.cc
[modify] https://crrev.com/c934896452fd0d3e518d1ffe77f68e0a25e66b34/ash/system/tray/system_tray.cc
[modify] https://crrev.com/c934896452fd0d3e518d1ffe77f68e0a25e66b34/ash/system/tray/system_tray.h
[modify] https://crrev.com/c934896452fd0d3e518d1ffe77f68e0a25e66b34/ash/system/tray/system_tray_delegate.cc
[modify] https://crrev.com/c934896452fd0d3e518d1ffe77f68e0a25e66b34/ash/system/tray/system_tray_delegate.h
[modify] https://crrev.com/c934896452fd0d3e518d1ffe77f68e0a25e66b34/ash/system/tray/system_tray_notifier.cc
[modify] https://crrev.com/c934896452fd0d3e518d1ffe77f68e0a25e66b34/ash/system/tray/system_tray_notifier.h
[modify] https://crrev.com/c934896452fd0d3e518d1ffe77f68e0a25e66b34/ash/test/test_system_tray_delegate.cc
[modify] https://crrev.com/c934896452fd0d3e518d1ffe77f68e0a25e66b34/ash/test/test_system_tray_delegate.h
[modify] https://crrev.com/c934896452fd0d3e518d1ffe77f68e0a25e66b34/chrome/browser/ui/ash/session_controller_client.cc
[modify] https://crrev.com/c934896452fd0d3e518d1ffe77f68e0a25e66b34/chrome/browser/ui/ash/session_controller_client.h
[modify] https://crrev.com/c934896452fd0d3e518d1ffe77f68e0a25e66b34/chrome/browser/ui/ash/session_controller_client_unittest.cc
[modify] https://crrev.com/c934896452fd0d3e518d1ffe77f68e0a25e66b34/chrome/browser/ui/ash/system_tray_delegate_chromeos.cc
[modify] https://crrev.com/c934896452fd0d3e518d1ffe77f68e0a25e66b34/chrome/browser/ui/ash/system_tray_delegate_chromeos.h