cros: Fix clusterfuzz crash when spawning app list very early in startup

ash/app_list/app_list_presenter_delegate.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/app_list/app_list_presenter_delegate.h"
 
 #include "ash/aura/wm_window_aura.h"
 #include "ash/common/ash_switches.h"
 #include "ash/common/shelf/app_list_button.h"
 #include "ash/common/shelf/shelf_layout_manager.h"
@@ skipping 95 matching lines @@
       ->GetShelfLayoutManager()
       ->UpdateAutoHideState();
   view_ = view;
   aura::Window* root_window = Shell::GetInstance()
                                   ->window_tree_host_manager()
                                   ->GetRootWindowForDisplayId(display_id);
   aura::Window* container = GetRootWindowController(root_window)
                                 ->GetContainer(kShellWindowId_AppListContainer);
   WmShelf* shelf = WmShelf::ForWindow(WmWindowAura::Get(container));
   AppListButton* applist_button = shelf->shelf_widget()->GetAppListButton();
+  DCHECK(applist_button);
   bool is_fullscreen = IsFullscreenAppListEnabled() &&
                        WmShell::Get()
                            ->maximize_mode_controller()
                            ->IsMaximizeModeWindowManagerEnabled();
   if (is_fullscreen) {
     view->InitAsFramelessWindow(
         container, current_apps_page,
         ScreenUtil::GetDisplayWorkAreaBoundsInParent(container));
   } else {
     view->InitAsBubble(container, current_apps_page);
@@ skipping 151 matching lines @@
 }
 
 ////////////////////////////////////////////////////////////////////////////////
 // AppListPresenterDelegate, WmShelfObserver implementation:
 
 void AppListPresenterDelegate::OnShelfIconPositionsChanged() {
   UpdateBounds();
 }
 
 }  // namespace ash