Update perf page_set to use story_set

tools/perf/page_sets/tough_animated_image_cases.py

 # Copyright 2015 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
+
 
 class ToughAnimatedImageCasesPage(page_module.Page):
 
   def __init__(self, url, page_set):
     super(ToughAnimatedImageCasesPage, self).__init__(
         url=url, page_set=page_set)
 
   def RunPageInteractions(self, action_runner):
     with action_runner.CreateInteraction('ToughAnimatedImage'):
       action_runner.Wait(10)
 
-class ToughAnimatedImageCasesPageSet(page_set_module.PageSet):
+class ToughAnimatedImageCasesPageSet(story.StorySet):
 
   """
   Description: A collection of difficult animated image tests
   """
 
   def __init__(self):
     super(ToughAnimatedImageCasesPageSet, self).__init__()
 
     urls_list = [
       'file://tough_animated_image_cases/gifs.html'
     ]
 
     for url in urls_list:
       self.AddUserStory(ToughAnimatedImageCasesPage(url, self))