I just created a mojo interface to the SystemTray. See ash/public/interfaces. It only has one method right now, but we could add more.

A portion of the current access is to synchronize the focus order for a11y.

We'll need to somehow synchronize chrome-ash focus ordering to account for this.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/328a830cd41918893878cad3a5e33299b1727150

commit 328a830cd41918893878cad3a5e33299b1727150
Author: jonross <jonross@chromium.org>
Date: Tue Jan 10 00:57:12 2017

Connect Login UI to SystemTray

WebuiLoginView toggles the visibility and enabled state of the SystemTray.
However when running in mash it cannot directly access the tray.

This change updates the system_tray.mojom to add the needed calls, and updates
WebuiLoginView to use the mojom for both normal and mash modes.

BUG= 648758 
TEST=ran associated browser tests, manual testing of login in normal and mash

Review-Url: https://codereview.chromium.org/2625473002
Cr-Commit-Position: refs/heads/master@{#442430}

[modify] https://crrev.com/328a830cd41918893878cad3a5e33299b1727150/ash/common/system/tray/system_tray_controller.cc
[modify] https://crrev.com/328a830cd41918893878cad3a5e33299b1727150/ash/common/system/tray/system_tray_controller.h
[modify] https://crrev.com/328a830cd41918893878cad3a5e33299b1727150/ash/public/interfaces/system_tray.mojom
[modify] https://crrev.com/328a830cd41918893878cad3a5e33299b1727150/chrome/browser/chromeos/login/ui/webui_login_view.cc
[modify] https://crrev.com/328a830cd41918893878cad3a5e33299b1727150/chrome/browser/ui/ash/system_tray_client.cc
[modify] https://crrev.com/328a830cd41918893878cad3a5e33299b1727150/chrome/browser/ui/ash/system_tray_client.h

 Issue 557401  for ash a11y blocks this feature. We need to sort out how ash a11y focus handling will interact with chrome ui.

This is done with  issue 854346 .
CL: https://chromium-review.googlesource.com/c/chromium/src/+/1325689