Update perf page_set to use story_set

tools/perf/page_sets/tough_layout_cases.py

 # 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.
+from telemetry import story
 from telemetry.page import page as page_module
-from telemetry.page import page_set as page_set_module
 from telemetry.page import shared_page_state
 
 
 class ToughLayoutCasesPage(page_module.Page):
 
   def __init__(self, url, page_set):
     super(ToughLayoutCasesPage, self).__init__(
         url=url, page_set=page_set, credentials_path = 'data/credentials.json',
         shared_page_state_class=shared_page_state.SharedDesktopPageState)
     self.archive_data_file = 'data/tough_layout_cases.json'
 
 
-class ToughLayoutCasesPageSet(page_set_module.PageSet):
+class ToughLayoutCasesPageSet(story.StorySet):
 
   """
   The slowest layouts observed in the alexa top 1 million sites in  July 2013.
   """
 
   def __init__(self):
     super(ToughLayoutCasesPageSet, self).__init__(
       archive_data_file='data/tough_layout_cases.json',
-      bucket=page_set_module.PARTNER_BUCKET)
+      cloud_storage_bucket=story.PARTNER_BUCKET)
 
     urls_list = [
       'http://oilevent.com',
       'http://www.muzoboss.ru',
       'http://natunkantha.com',
       'http://www.mossiella.com',
       'http://bookish.com',
       'http://mydiyclub.com',
       'http://amarchoti.blogspot.com',
       'http://picarisimo.es',
       'http://chinaapache.com',
       'http://indoritel.com'
     ]
 
     for url in urls_list:
       self.AddUserStory(ToughLayoutCasesPage(url, self))