Update perf page_set to use story_set

tools/perf/page_sets/tough_memory_multi_tab.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 shared_page_state
-from telemetry.page import page as page_module
-from telemetry.page import page_set as page_set_module
 
 
 class ToughMemoryMultiTabPage(page_module.Page):
 
   def __init__(self, url, page_set):
     super(ToughMemoryMultiTabPage, self).__init__(
         url=url, page_set=page_set, credentials_path = 'data/credentials.json',
         shared_page_state_class=shared_page_state.SharedMobilePageState)
     self.archive_data_file = 'data/key_mobile_sites.json'
 
 
-class ToughMemoryMultiTabPageSet(page_set_module.PageSet):
+class ToughMemoryMultiTabPageSet(story.StorySet):
 
   """ Mobile sites for exercising multi-tab memory issues """
 
   def __init__(self):
     super(ToughMemoryMultiTabPageSet, self).__init__(
       archive_data_file='data/key_mobile_sites.json')
 
     urls_list = [
       'https://www.google.com/#hl=en&q=barack+obama',
       'http://theverge.com',
       'http://techcrunch.com'
     ]
 
     for url in urls_list:
       self.AddUserStory(ToughMemoryMultiTabPage(url, self))