Update perf page_set to use story_set

tools/perf/page_sets/tough_scrolling_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.internal.actions import page_action
 from telemetry.page import page as page_module
-from telemetry.page import page_set as page_set_module
 
 
 class ToughFastScrollingCasesPage(page_module.Page):
 
   def __init__(self, url, name, speed_in_pixels_per_second, page_set,
                synthetic_gesture_source):
     super(ToughFastScrollingCasesPage, self).__init__(
       url=url,
       page_set=page_set,
       name=name)
     self.speed_in_pixels_per_second = speed_in_pixels_per_second
     self.synthetic_gesture_source = synthetic_gesture_source
 
   def RunPageInteractions(self, action_runner):
     with action_runner.CreateGestureInteraction('ScrollAction'):
       action_runner.ScrollPage(
           direction='down',
           speed_in_pixels_per_second=self.speed_in_pixels_per_second,
           synthetic_gesture_source=self.synthetic_gesture_source)
 
-class ToughScrollingCasesPageSet(page_set_module.PageSet):
+class ToughScrollingCasesPageSet(story.StorySet):
 
   """
   Description: A collection of difficult scrolling tests
   """
 
   def __init__(self):
     super(ToughScrollingCasesPageSet, self).__init__()
 
     fast_scrolling_page_name_list = [
       'text',
       'text_hover',
       'canvas'
     ]
 
     fast_scrolling_speed_list = [
       5000, 10000, 15000, 20000, 30000, 40000, 50000, 60000, 75000, 90000
     ]
 
     for name in fast_scrolling_page_name_list:
       for speed in fast_scrolling_speed_list:
         synthetic_gesture_source = page_action.GESTURE_SOURCE_DEFAULT
         if "hover" in name:
           synthetic_gesture_source = page_action.GESTURE_SOURCE_MOUSE
         self.AddUserStory(ToughFastScrollingCasesPage(
           'file://tough_scrolling_cases/' + name + '.html',
           name + '_' + str(speed).zfill(5) + '_pixels_per_second',
           speed,
           self,
           synthetic_gesture_source))