Fix 'except' and 'prefer' spelling.

chrome/browser/ui/ash/multi_user/user_switch_animator_chromeos.cc

 // Copyright 2014 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 "chrome/browser/ui/ash/multi_user/user_switch_animator_chromeos.h"
 
 #include "ash/desktop_background/user_wallpaper_delegate.h"
 #include "ash/root_window_controller.h"
 #include "ash/shelf/shelf_layout_manager.h"
 #include "ash/shell.h"
@@ skipping 168 matching lines @@
         duration_override);
   }
 
   // For each root window hide the shelf.
   if (animation_step == ANIMATION_STEP_HIDE_OLD_USER) {
     aura::Window::Windows root_windows = ash::Shell::GetAllRootWindows();
     for (aura::Window::Windows::const_iterator iter = root_windows.begin();
          iter != root_windows.end(); ++iter) {
       // This shelf change is only part of the animation and will be updated by
       // ChromeLauncherController::ActiveUserChanged() to the new users value.
-      // Note that the user perference will not be changed.
+      // Note that the user preference will not be changed.
       ash::Shell::GetInstance()->SetShelfAutoHideBehavior(
           ash::SHELF_AUTO_HIDE_ALWAYS_HIDDEN, *iter);
     }
   }
 }
 
 void UserSwichAnimatorChromeOS::TransitionWindows(
     AnimationStep animation_step) {
   // Disable the window position manager and the MRU window tracker temporarily.
   UserChangeActionDisabler disabler;
@@ skipping 117 matching lines @@
       if (cover == OLD_USER_COVERS_SCREEN)
         return BOTH_USERS_COVER_SCREEN;
       else
         cover = NEW_USER_COVERS_SCREEN;
     }
   }
   return cover;
 }
 
 }  // namespace chrome