Update perf page_set to use story_set

tools/perf/page_sets/blank_page.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 BlankPage(page_module.Page):
   def __init__(self, url, page_set):
     super(BlankPage, self).__init__(url, page_set=page_set)
 
 
-class BlankPageSet(page_set_module.PageSet):
+class BlankPageSet(story.StorySet):
   """A single blank page."""
 
   def __init__(self):
     super(BlankPageSet, self).__init__()
     self.AddUserStory(BlankPage('file://blank_page/blank_page.html', self))