Stop the OverviewButton from appearing on the Add-User screen

ash/system/overview/overview_button_tray.cc

Index: ash/system/overview/overview_button_tray.cc
diff --git a/ash/system/overview/overview_button_tray.cc b/ash/system/overview/overview_button_tray.cc
index edd5d67dd0697dd2598133cc5289d83e7511831f..3b34aea30e90074bcd985fafa5ea70492f8c7892 100644
--- a/ash/system/overview/overview_button_tray.cc
+++ b/ash/system/overview/overview_button_tray.cc
@@ -4,6 +4,7 @@
 
 #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"
@@ -42,10 +43,13 @@ OverviewButtonTray::OverviewButtonTray(StatusAreaWidget* status_area_widget)
   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(
@@ -62,6 +66,11 @@ bool OverviewButtonTray::PerformAction(const ui::Event& event) {
   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();
 }