I think we're going to need to add a mojo method to system_tray.mojom. update_engine doesn't broadcast signals for some of the cellular-download state changes, so it's hard to have multiple dbus clients listen to them.

It looks like this code is only used to hide a row in the system tray menu when the user responds to the "do you want to download over cellular" webui dialog. (Does the row always go away, or only if the user says no? Says yes?)

It might be possible to change mojom::SystemTray::ShowUpdateOverCellularAvailableIcon() to take a "bool visible" parameter, but I'm not sure if calling that with false is semantically equivalent to OnUpdateOverCellularTargetSet(true).

Right now, SystemTrayDelegateChromeOS is an observer of UpdateEngineClient. And it triggers the icon disappear in OnUpdateOverCellularTargetSet.
I could make a change using the same path as update icon shows up in system tray instead.

Old path: UpdateEngineClient -> SystemTrayDelegateChromeOS::OnUpdateOverCellularTargetSet -> SystemTrayNotifier::NotifyUpdateOverCellularTargetSet -> TrayUpdate::OnUpdateOverCellularTargetSet

New path: UpdateEngineClient -> UpgradeDetectorChromeos::OnUpdateOverCellularTargetSet -> SystemTrayClient::NotifyUpdateOverCellularTargetSet -> 
mojom::SystemTray::ShowUpdateOverCellularAvailableIcon(false) ->
TrayUpdate::ShowUpdateOverCellularAvailableIcon(false)

Does that make sense?

I'll have a draft CL available soon.

Draft CL available at https://chromium-review.googlesource.com/578062

Feel free to take over that CL, or throw it away and do something different. The important part is eliminating the code in SystemTrayDelegateChromeOS and only sending data to ash over mojo.

Your approach in comment #3 sounds reasonable. You'll need to be sure ShowUpdateOverCellularAvailableIcon(false) is what you want. I don't know if we want the tray icon to disappear while downloading the update, or just to hide the row in the menu. Since I don't have a good way to test this feature on a device I don't know how it behaves in production.

Ok, I would take over the CL and implement the approach in #3. The tray icon disappears when a setting is successfully enabled in Update Engine. So It should be before downloading begins. Thanks.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/8f8b70016a2958cc84e4f10cfe53e193be61cc37

commit 8f8b70016a2958cc84e4f10cfe53e193be61cc37
Author: Weidong Guo <weidongg@chromium.org>
Date: Fri Jul 28 19:37:02 2017

cros: Support update-over-cellular for mash

For go/mustash code in chrome browser and code in ash run in separate
processes, so code in chrome must use mojo interfaces to send data to
ash.

Migrate update-over-cellular code from SystemTrayDelegateChromeOS (where
it calls directly into ash) to SystemTrayClient (which uses mojo).

Eliminate some layers of observers in ash.

Change FakeUpdateEngineClient to support testing cellular updates.

Add test coverage.

BUG= 746574 ,  745975 
TEST=SystemTrayClientTest.CellularUpdateTrayIcon

Change-Id: I9a908e08011fed08204ba7fdea31a27d00a7e552
Reviewed-on: https://chromium-review.googlesource.com/583558
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Reviewed-by: James Cook <jamescook@chromium.org>
Reviewed-by: Michael Wasserman <msw@chromium.org>
Reviewed-by: Steven Bennetts <stevenjb@chromium.org>
Commit-Queue: Weidong Guo <weidongg@chromium.org>
Cr-Commit-Position: refs/heads/master@{#490487}
[modify] https://crrev.com/8f8b70016a2958cc84e4f10cfe53e193be61cc37/ash/public/interfaces/system_tray.mojom
[modify] https://crrev.com/8f8b70016a2958cc84e4f10cfe53e193be61cc37/ash/system/tray/system_tray_controller.cc
[modify] https://crrev.com/8f8b70016a2958cc84e4f10cfe53e193be61cc37/ash/system/tray/system_tray_controller.h
[modify] https://crrev.com/8f8b70016a2958cc84e4f10cfe53e193be61cc37/ash/system/tray/system_tray_notifier.cc
[modify] https://crrev.com/8f8b70016a2958cc84e4f10cfe53e193be61cc37/ash/system/tray/system_tray_notifier.h
[modify] https://crrev.com/8f8b70016a2958cc84e4f10cfe53e193be61cc37/ash/system/update/tray_update.cc
[modify] https://crrev.com/8f8b70016a2958cc84e4f10cfe53e193be61cc37/ash/system/update/tray_update.h
[modify] https://crrev.com/8f8b70016a2958cc84e4f10cfe53e193be61cc37/ash/system/update/tray_update_unittest.cc
[delete] https://crrev.com/889b291c4123f1e38f49327aa67026b894964d83/ash/system/update/update_observer.h
[modify] https://crrev.com/8f8b70016a2958cc84e4f10cfe53e193be61cc37/chrome/browser/chromeos/upgrade_detector_chromeos.cc
[modify] https://crrev.com/8f8b70016a2958cc84e4f10cfe53e193be61cc37/chrome/browser/chromeos/upgrade_detector_chromeos.h
[modify] https://crrev.com/8f8b70016a2958cc84e4f10cfe53e193be61cc37/chrome/browser/ui/ash/system_tray_client.cc
[modify] https://crrev.com/8f8b70016a2958cc84e4f10cfe53e193be61cc37/chrome/browser/ui/ash/system_tray_client.h
[modify] https://crrev.com/8f8b70016a2958cc84e4f10cfe53e193be61cc37/chrome/browser/ui/ash/system_tray_client_browsertest.cc
[modify] https://crrev.com/8f8b70016a2958cc84e4f10cfe53e193be61cc37/chrome/browser/ui/ash/system_tray_delegate_chromeos.cc
[modify] https://crrev.com/8f8b70016a2958cc84e4f10cfe53e193be61cc37/chrome/browser/ui/ash/system_tray_delegate_chromeos.h
[modify] https://crrev.com/8f8b70016a2958cc84e4f10cfe53e193be61cc37/chrome/browser/ui/webui/help/version_updater.h
[modify] https://crrev.com/8f8b70016a2958cc84e4f10cfe53e193be61cc37/chrome/browser/ui/webui/help/version_updater_chromeos.cc
[modify] https://crrev.com/8f8b70016a2958cc84e4f10cfe53e193be61cc37/chrome/browser/ui/webui/help/version_updater_chromeos.h
[modify] https://crrev.com/8f8b70016a2958cc84e4f10cfe53e193be61cc37/chrome/browser/ui/webui/settings/about_handler.cc
[modify] https://crrev.com/8f8b70016a2958cc84e4f10cfe53e193be61cc37/chrome/browser/ui/webui/settings/about_handler.h
[modify] https://crrev.com/8f8b70016a2958cc84e4f10cfe53e193be61cc37/chrome/browser/upgrade_detector.cc
[modify] https://crrev.com/8f8b70016a2958cc84e4f10cfe53e193be61cc37/chrome/browser/upgrade_detector.h
[modify] https://crrev.com/8f8b70016a2958cc84e4f10cfe53e193be61cc37/chrome/browser/upgrade_observer.h
[modify] https://crrev.com/8f8b70016a2958cc84e4f10cfe53e193be61cc37/chromeos/dbus/fake_update_engine_client.cc
[modify] https://crrev.com/8f8b70016a2958cc84e4f10cfe53e193be61cc37/chromeos/dbus/fake_update_engine_client.h
[modify] https://crrev.com/8f8b70016a2958cc84e4f10cfe53e193be61cc37/chromeos/dbus/update_engine_client.cc
[modify] https://crrev.com/8f8b70016a2958cc84e4f10cfe53e193be61cc37/chromeos/dbus/update_engine_client.h

I would mark this as fixed, please feel free to reopen it.