Update perf page_set to use story_set

tools/perf/page_sets/tough_image_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 shared_page_state
-from telemetry.page import page as page_module
-from telemetry.page import page_set as page_set_module
 
 
 class ToughImageCasesPage(page_module.Page):
 
   def __init__(self, url, page_set):
     super(ToughImageCasesPage, self).__init__(
         url=url, page_set=page_set,
         shared_page_state_class=shared_page_state.SharedDesktopPageState)
 
 
-class ToughImageCasesPageSet(page_set_module.PageSet):
+class ToughImageCasesPageSet(story.StorySet):
 
   """ A collection of image-heavy sites. """
 
   def __init__(self):
     super(ToughImageCasesPageSet, self).__init__()
 
     urls_list = [
       'http://www.free-pictures-photos.com/aviation/airplane-306.jpg',
       ('http://upload.wikimedia.org/wikipedia/commons/c/cb/'
        'General_history%2C_Alaska_Yukon_Pacific_Exposition%'
        '2C_fully_illustrated_-_meet_me_in_Seattle_1909_-_Page_78.jpg')
     ]
 
     for url in urls_list:
       self.AddUserStory(ToughImageCasesPage(url, self))