I will take the chance to consolidate the LoginStatus code path:
- Make cash/mash use the same code path for LoginStatus update;
- Migrate ash code to use only SessionController::GetLoginStatus;

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/233f4e71bd49e79e477ef3ba814d53e8036e3c50

commit 233f4e71bd49e79e477ef3ba814d53e8036e3c50
Author: xiyuan <xiyuan@chromium.org>
Date: Thu Apr 06 06:59:56 2017

ash: Merge LoginStatus update code path

- Add LoginStatusChanged to SessionStateObserver;
- Make SessionController fire LoginStatusChanged;
- Shell call UpdateAfterLoginStatusChange on LoginStatusChanged;
- Remove Shell::OnLoginStateChanged and notify ShellObserver in
  Shell::LoginStatusChanged;
- Remove UpdateAfterLoginStatusChange calls from SystemTrayDelegateChromeOS
  and TestSessionControllerClient;
- Remove Shell::OnLoginStateChanged calls from LoginLockStateNotifier
  since LoginStatusChanged should cover that;
- Fix ash_shell_with_content that SessionController is not bound
  to its client;

BUG=701193, 679450 

Review-Url: https://codereview.chromium.org/2797743002
Cr-Commit-Position: refs/heads/master@{#462378}

[modify] https://crrev.com/233f4e71bd49e79e477ef3ba814d53e8036e3c50/ash/common/session/session_controller.cc
[modify] https://crrev.com/233f4e71bd49e79e477ef3ba814d53e8036e3c50/ash/common/session/session_controller.h
[modify] https://crrev.com/233f4e71bd49e79e477ef3ba814d53e8036e3c50/ash/common/session/session_controller_unittest.cc
[modify] https://crrev.com/233f4e71bd49e79e477ef3ba814d53e8036e3c50/ash/common/session/session_state_observer.h
[modify] https://crrev.com/233f4e71bd49e79e477ef3ba814d53e8036e3c50/ash/common/system/user/tray_user.cc
[modify] https://crrev.com/233f4e71bd49e79e477ef3ba814d53e8036e3c50/ash/common/test/test_session_controller_client.cc
[modify] https://crrev.com/233f4e71bd49e79e477ef3ba814d53e8036e3c50/ash/mus/system_tray_delegate_mus.cc
[modify] https://crrev.com/233f4e71bd49e79e477ef3ba814d53e8036e3c50/ash/shell.cc
[modify] https://crrev.com/233f4e71bd49e79e477ef3ba814d53e8036e3c50/ash/shell.h
[modify] https://crrev.com/233f4e71bd49e79e477ef3ba814d53e8036e3c50/ash/shell/content/client/shell_browser_main_parts.cc
[modify] https://crrev.com/233f4e71bd49e79e477ef3ba814d53e8036e3c50/ash/shell/example_session_controller_client.cc
[modify] https://crrev.com/233f4e71bd49e79e477ef3ba814d53e8036e3c50/ash/wm/lock_state_controller.cc
[modify] https://crrev.com/233f4e71bd49e79e477ef3ba814d53e8036e3c50/ash/wm/lock_state_controller.h
[modify] https://crrev.com/233f4e71bd49e79e477ef3ba814d53e8036e3c50/chrome/browser/chromeos/power/login_lock_state_notifier.cc
[modify] https://crrev.com/233f4e71bd49e79e477ef3ba814d53e8036e3c50/chrome/browser/chromeos/system/tray_accessibility_browsertest.cc
[modify] https://crrev.com/233f4e71bd49e79e477ef3ba814d53e8036e3c50/chrome/browser/ui/ash/session_controller_client.cc
[modify] https://crrev.com/233f4e71bd49e79e477ef3ba814d53e8036e3c50/chrome/browser/ui/ash/session_controller_client.h
[modify] https://crrev.com/233f4e71bd49e79e477ef3ba814d53e8036e3c50/chrome/browser/ui/ash/system_tray_delegate_chromeos.cc
[modify] https://crrev.com/233f4e71bd49e79e477ef3ba814d53e8036e3c50/chrome/browser/ui/ash/system_tray_delegate_chromeos.h