Update perf page_set to use story_set

tools/perf/page_sets/image_decoding_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 ImageDecodingCasesPage(page_module.Page):
 
   def __init__(self, url, page_set):
     super(ImageDecodingCasesPage, self).__init__(url=url, page_set=page_set)
 
   def RunPageInteractions(self, action_runner):
     with action_runner.CreateInteraction('DecodeImage'):
       action_runner.Wait(5)
 
-class ImageDecodingCasesPageSet(page_set_module.PageSet):
+class ImageDecodingCasesPageSet(story.StorySet):
 
   """ A directed benchmark of accelerated jpeg image decoding performance """
 
   def __init__(self):
     super(ImageDecodingCasesPageSet, self).__init__()
 
     urls_list = [
       'file://image_decoding_cases/yuv_decoding.html'
     ]
 
     for url in urls_list:
       self.AddUserStory(ImageDecodingCasesPage(url, self))