Update perf page_set to use story_set

tools/perf/page_sets/alexa1-10000.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.
 import json
 import os
 
+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
 
-
 __location__ = os.path.realpath(
       os.path.join(os.getcwd(), os.path.dirname(__file__)))
 
 # Generated on 2013-09-03 13:59:53.459117 by rmistry using
 # create_page_set.py.
 _TOP_10000_ALEXA_FILE = os.path.join(__location__, 'alexa1-10000-urls.json')
 
 
 class Alexa1To10000Page(page_module.Page):
 
   def __init__(self, url, page_set):
     super(Alexa1To10000Page, self).__init__(
         url=url, page_set=page_set,
         shared_page_state_class=shared_page_state.SharedDesktopPageState)
 
   def RunPageInteractions(self, action_runner):
     with action_runner.CreateGestureInteraction('ScrollAction'):
       action_runner.ScrollPage()
 
 
-class Alexa1To10000PageSet(page_set_module.PageSet):
+class Alexa1To10000PageSet(story.StorySet):
   """ Top 1-10000 Alexa global.
       Generated on 2013-09-03 13:59:53.459117 by rmistry using
       create_page_set.py.
   """
 
   def __init__(self):
     super(Alexa1To10000PageSet, self).__init__()
 
     with open(_TOP_10000_ALEXA_FILE) as f:
       urls_list = json.load(f)
     for url in urls_list:
       self.AddUserStory(Alexa1To10000Page(url, self))