Stop the OverviewButton from appearing on the Add-User screen

ash/system/overview/overview_button_tray.cc

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "ash/system/overview/overview_button_tray.h"
 
+#include "ash/session/session_state_delegate.h"
 #include "ash/shelf/shelf_types.h"
 #include "ash/shell.h"
 #include "ash/system/tray/system_tray_delegate.h"
 #include "ash/system/tray/tray_utils.h"
 #include "ash/wm/maximize_mode/maximize_mode_controller.h"
 #include "ash/wm/overview/window_selector_controller.h"
 #include "grit/ash_resources.h"
 #include "grit/ash_strings.h"
 #include "ui/base/l10n/l10n_util.h"
 #include "ui/base/resource/resource_bundle.h"
@@ skipping 18 matching lines @@
   SetContentsBackground();
 
   ui::ResourceBundle& bundle = ui::ResourceBundle::GetSharedInstance();
   icon_ = new views::ImageView();
   icon_->SetImage(
       bundle.GetImageNamed(IDR_AURA_UBER_TRAY_OVERVIEW_MODE).ToImageSkia());
   SetIconBorderForShelfAlignment();
   tray_container()->AddChildView(icon_);
 
   Shell::GetInstance()->AddShellObserver(this);
+  Shell::GetInstance()->session_state_delegate()->AddSessionStateObserver(this);
 }
 
 OverviewButtonTray::~OverviewButtonTray() {
   Shell::GetInstance()->RemoveShellObserver(this);
+  Shell::GetInstance()->session_state_delegate()->RemoveSessionStateObserver(
+      this);
 }
 
 void OverviewButtonTray::UpdateAfterLoginStatusChange(
     user::LoginStatus status) {
   UpdateIconVisibility();
 }
 
 bool OverviewButtonTray::PerformAction(const ui::Event& event) {
   WindowSelectorController* controller =
       Shell::GetInstance()->window_selector_controller();
   controller->ToggleOverview();
   SetDrawBackgroundAsActive(controller->IsSelecting());
   Shell::GetInstance()->metrics()->RecordUserMetricsAction(UMA_TRAY_OVERVIEW);
   return true;
 }
 
+void OverviewButtonTray::SessionStateChanged(
+    SessionStateDelegate::SessionState state) {
+  UpdateIconVisibility();

[flackr, 2015/05/28 15:20:37]

I think this is getting brittle, the logic / responsibility for determining when
we can enter overview being split between window_selector_controller to query
and any class which wants to know about when this state changes to observe all
possible events which cause the state to change.

Can you move the necessary observers to determine when CanSelect changes to
window_selector_controller and make it so that this class only needs to observe
something on window_selector_controller? (And still observe maximize mode
because that is independent of whether CanSelect is true)

[jonross, 2015/06/01 17:00:37]

I've decided to decouple OverviewButton's visibility from WindowSelectors
ability to activate.

As seen in:
https://codereview.chromium.org/1114383002/
there is another case where the two are separate.

I've updated OverviewButtonTray to track the aspects of user sessions to
determine when to show.

+}
+
 void OverviewButtonTray::OnMaximizeModeStarted() {
   UpdateIconVisibility();
 }
 
 void OverviewButtonTray::OnMaximizeModeEnded() {
   UpdateIconVisibility();
 }
 
 void OverviewButtonTray::OnOverviewModeEnded() {
   SetDrawBackgroundAsActive(false);
@@ skipping 39 matching lines @@
   }
 }
 
 void OverviewButtonTray::UpdateIconVisibility() {
   SetVisible(Shell::GetInstance()->maximize_mode_controller()->
                  IsMaximizeModeWindowManagerEnabled() &&
              Shell::GetInstance()->window_selector_controller()->CanSelect());
 }
 
 }  // namespace ash