Update perf page_set to use story_set

tools/perf/page_sets/blank_page_with_large_profile.py

 # Copyright 2015 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 page_sets import pregenerated_large_profile_shared_state
+from telemetry import story
 from telemetry.page import page as page_module
-from telemetry.page import page_set as page_set_module
 
 
 class BlankPageWithLargeProfile(page_module.Page):
   def __init__(self, url, page_set):
     super(BlankPageWithLargeProfile, self).__init__(
         url=url, page_set=page_set,
         shared_page_state_class=pregenerated_large_profile_shared_state.
         PregeneratedLargeProfileSharedState)
 
 
-class BlankPageSetWithLargeProfile(page_set_module.PageSet):
+class BlankPageSetWithLargeProfile(story.StorySet):
   """A single blank page loaded with a large profile."""
 
   def __init__(self):
     super(BlankPageSetWithLargeProfile, self).__init__()
     self.AddUserStory(BlankPageWithLargeProfile(
         'file://blank_page/blank_page.html', self))