Fix Chrome crashes on calling OnTrayVisibilityChange

ash/common/system/status_area_widget.cc

Index: ash/common/system/status_area_widget.cc
diff --git a/ash/common/system/status_area_widget.cc b/ash/common/system/status_area_widget.cc
index 06bdf90c0d2c21eceda997e746f013cf09ae213d..7d69835501563da6c5d3ccd133454acac25a8f7b 100644
--- a/ash/common/system/status_area_widget.cc
+++ b/ash/common/system/status_area_widget.cc
@@ -282,10 +282,11 @@ void StatusAreaWidget::AddImeMenuTray() {
 
 bool StatusAreaWidget::IsNextVisibleTrayToLogout(
     TrayBackgroundView* tray) const {
+  if (!logout_button_tray_)

[Daniele Castagna, 2016/12/13 17:24:58]

Is whoever needs this going to call IsNextVisibleTrayToLogout again after it
fails the first time?

+    return false;
+
   int logout_button_index =
       status_area_widget_delegate_->GetIndexOf(logout_button_tray_);
-  // Logout button should always exist.
-  DCHECK_NE(-1, logout_button_index);
   if (!logout_button_tray_->visible())
     return false;