Update perf page_set to use story_set

tools/perf/page_sets/startup_pages_record.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 StartupPagesRecordPage(page_module.Page):
 
   def __init__(self, url, page_set):
     super(StartupPagesRecordPage, self).__init__(url=url, page_set=page_set)
     self.archive_data_file = 'data/startup_pages.json'
 
 
-class StartupPagesRecordPageSet(page_set_module.PageSet):
+class StartupPagesRecordPageSet(story.StorySet):
 
   """ Pages to record data for testing starting Chrome with a URL.
       We can't use startup_pages.json with record_wpr, since record_wpr
       requires a default navigate step, which we don't want for startup
       testing; but we do want to record the pages it uses. Also, record_wpr
       fails on about:blank, which we want to include in startup testing.
   """
 
   def __init__(self):
     super(StartupPagesRecordPageSet, self).__init__(
         archive_data_file='data/startup_pages.json')
 
     urls_list = [
         # Why: typical page
         'http://bbc.co.uk',
         # Why: Horribly complex page - stress test!
         'http://kapook.com',
     ]
 
     for url in urls_list:
       self.AddUserStory(StartupPagesRecordPage(url, self))