Update perf page_set to use story_set

tools/perf/page_sets/media_cns_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 BasicPlayPage(page_module.Page):
 
   def __init__(self, url, page_set):
     super(BasicPlayPage, self).__init__(url=url, page_set=page_set)
     self.add_browser_metrics = True
 
   def PlayAction(self, action_runner):
     action_runner.PlayMedia(playing_event_timeout_in_seconds=60,
@@ skipping 17 matching lines @@
 
   def __init__(self, url, page_set):
     super(SeekBeforeAndAfterPlayheadPage, self).__init__(url=url,
                                                          page_set=page_set)
     self.add_browser_metrics = False
 
   def RunPageInteractions(self, action_runner):
     self.SeekBeforeAndAfterPlayhead(action_runner)
 
 
-class MediaCnsCasesPageSet(page_set_module.PageSet):
+class MediaCnsCasesPageSet(story.StorySet):
 
   """ Media benchmark on network constrained conditions. """
 
   def __init__(self):
     super(MediaCnsCasesPageSet, self).__init__()
 
     urls_list = [
       # pylint: disable=C0301
       'file://tough_video_cases/video.html?id=no_constraints_webm&src=tulip2.webm&net=none',
       # pylint: disable=C0301
@@ skipping 53 matching lines @@
       # pylint: disable=C0301
       'file://tough_video_cases/video.html?id=wifi_mp4&src=tulip2.mp4&type=audio&net=wifi',
       # pylint: disable=C0301
       'file://tough_video_cases/video.html?id=wifi_ogv&src=tulip2.ogv&type=audio&net=wifi',
       # pylint: disable=C0301
       'file://tough_video_cases/video.html?id=wifi_webm&src=tulip2.webm&type=audio&net=wifi'
     ]
 
     for url in urls_list2:
       self.AddUserStory(SeekBeforeAndAfterPlayheadPage(url, self))