Fix Chrome crashes on calling OnTrayVisibilityChange

ash/common/system/status_area_widget.cc

 // Copyright (c) 2012 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/common/system/status_area_widget.h"
 
 #include "ash/common/material_design/material_design_controller.h"
 #include "ash/common/shelf/wm_shelf.h"
 #include "ash/common/system/overview/overview_button_tray.h"
 #include "ash/common/system/status_area_widget_delegate.h"
@@ skipping 264 matching lines @@
   status_area_widget_delegate_->AddTray(virtual_keyboard_tray_);
 }
 
 void StatusAreaWidget::AddImeMenuTray() {
   ime_menu_tray_ = new ImeMenuTray(wm_shelf_);
   status_area_widget_delegate_->AddTray(ime_menu_tray_);
 }
 
 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;
 
   for (int c = logout_button_index + 1;
        c < status_area_widget_delegate_->child_count(); c++) {
     if (status_area_widget_delegate_->child_at(c)->visible())
       return tray == status_area_widget_delegate_->child_at(c);
   }
   return false;
 }
 #endif
 
 void StatusAreaWidget::AddOverviewButtonTray() {
   overview_button_tray_ = new OverviewButtonTray(wm_shelf_);
   status_area_widget_delegate_->AddTray(overview_button_tray_);
 }
 
 }  // namespace ash