Added back button when in folder view.

ui/app_list/views/app_list_view.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 "ui/app_list/views/app_list_view.h"
 
 #include <algorithm>
 
 #include "base/command_line.h"
 #include "base/macros.h"
@@ skipping 830 matching lines @@
         }
         case FULLSCREEN_SEARCH:
         case FULLSCREEN_ALL_APPS:
         case CLOSED:
           NOTREACHED();
           break;
       }
       break;
     case FULLSCREEN_ALL_APPS:
       new_widget_bounds.set_y(0);
+      if (app_list_main_view()
+              ->contents_view()
+              ->apps_container_view()
+              ->IsInFolderView()) {
+        app_list_main_view()
+            ->contents_view()
+            ->apps_container_view()

[vadimt, 2017/06/22 18:28:08]

Actually, please create a var for the common part: app_list_main_view()
             ->contents_view()
            ->apps_container_view()

[newcomer, 2017/06/23 23:28:08]

Done.

+            ->app_list_folder_view()
+            ->CloseFolderPage();
+      }
       app_list_main_view_->contents_view()->SetActiveState(
           AppListModel::STATE_APPS);
       break;
     case FULLSCREEN_SEARCH:
       new_widget_bounds.set_y(0);
       break;
     case CLOSED:
       app_list_main_view_->Close();
       delegate_->Dismiss();
       break;
@@ skipping 97 matching lines @@
                        .work_area()
                        .size();
   size.Enlarge(0, kShelfSize);
   fullscreen_widget_->SetSize(size);
 
   // Update the |fullscreen_widget_| bounds to accomodate the new work area.
   SetState(app_list_state_);
 }
 
 }  // namespace app_list