Update perf page_set to use story_set

tools/perf/page_sets/five_blank_pages.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
 
 
 # The PageSet searches for pages relative to the directory the page class is
 # defined in so we need to subclass here.
 class BlankPage(page_module.Page):
   pass
 
 
-class FiveBlankPagesPageSet(page_set_module.PageSet):
+class FiveBlankPagesPageSet(story.StorySet):
 
   """ Five blank pages. """
 
   def __init__(self):
     super(FiveBlankPagesPageSet, self).__init__()
     for _ in xrange(5):
       self.AddUserStory(BlankPage('file://blank_page/blank_page.html', self))