Update perf page_set to use story_set

tools/perf/page_sets/tough_texture_upload_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
 
 
 class ToughTextureUploadCasesPage(page_module.Page):
 
   def __init__(self, url, page_set):
     super(
       ToughTextureUploadCasesPage,
       self).__init__(
         url=url,
         page_set=page_set)
 
   def RunPageInteractions(self, action_runner):
     with action_runner.CreateInteraction('Animation'):
       action_runner.Wait(10)
 
 
-class ToughTextureUploadCasesPageSet(page_set_module.PageSet):
+class ToughTextureUploadCasesPageSet(story.StorySet):
 
   """
   Description: A collection of texture upload performance tests
   """
 
   def __init__(self):
     super(ToughTextureUploadCasesPageSet, self).__init__()
 
     urls_list = [
       'file://tough_texture_upload_cases/background_color_animation.html',
       # pylint: disable=C0301
       'file://tough_texture_upload_cases/background_color_animation_and_transform_animation.html',
       # pylint: disable=C0301
       'file://tough_texture_upload_cases/background_color_animation_with_gradient.html',
       # pylint: disable=C0301
       'file://tough_texture_upload_cases/background_color_animation_with_gradient_and_transform_animation.html',
       'file://tough_texture_upload_cases/small_texture_uploads.html',
       'file://tough_texture_upload_cases/medium_texture_uploads.html',
       'file://tough_texture_upload_cases/large_texture_uploads.html',
       'file://tough_texture_upload_cases/extra_large_texture_uploads.html',
     ]
     for url in urls_list:
       self.AddUserStory(ToughTextureUploadCasesPage(url, self))
-