Overview Tray Button should not be visible on login screen

ash/wm/overview/window_selector_controller.cc

Index: ash/wm/overview/window_selector_controller.cc
diff --git a/ash/wm/overview/window_selector_controller.cc b/ash/wm/overview/window_selector_controller.cc
index 25419c4a6d2df55cc0eee001c4ebbbcab653746b..71b97bfb5bfcee6b7a82ba0d31285943c905d717 100644
--- a/ash/wm/overview/window_selector_controller.cc
+++ b/ash/wm/overview/window_selector_controller.cc
@@ -27,7 +27,9 @@ WindowSelectorController::~WindowSelectorController() {
 bool WindowSelectorController::CanSelect() {
   // Don't allow a window overview if the screen is locked or a modal dialog is
   // open.
-  return !Shell::GetInstance()->session_state_delegate()->IsScreenLocked() &&
+  return Shell::GetInstance()->session_state_delegate()->
+             IsActiveUserSessionStarted() &&
+         !Shell::GetInstance()->session_state_delegate()->IsScreenLocked() &&
          !Shell::GetInstance()->IsSystemModalWindowOpen();
 }