Update perf page_set to use story_set

tools/perf/page_sets/top_7_stress.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 shared_page_state
-from telemetry.page import page as page_module
-from telemetry.page import page_set as page_set_module
 
 
 def _GetCurrentLocation(action_runner):
   return action_runner.EvaluateJavaScript('document.location.href')
 
 
 def _WaitForLocationChange(action_runner, old_href):
   action_runner.WaitForJavaScriptCondition(
       'document.location.href != "%s"' % old_href)
 
@@ skipping 282 matching lines @@
         'document.documentElement.scrollHeight - window.innerHeight - '
         'window.pageYOffset > 0')
 
     # Scroll and wait again.
     with action_runner.CreateGestureInteraction('ScrollAction'):
       action_runner.ScrollPage()
     action_runner.WaitForJavaScriptCondition(
         'document.documentElement.scrollHeight - window.innerHeight - '
         'window.pageYOffset > 0')
 
-class Top7StressPageSet(page_set_module.PageSet):
+class Top7StressPageSet(story.StorySet):
 
   """ Pages hand-picked for stress testing. """
 
   def __init__(self):
     super(Top7StressPageSet, self).__init__(
       archive_data_file='data/top_7_stress.json',
-      bucket=page_set_module.PARTNER_BUCKET)
+      cloud_storage_bucket=story.PARTNER_BUCKET)
 
     self.AddUserStory(GoogleWebSearchPage(self))
     self.AddUserStory(GmailPage(self))
     self.AddUserStory(GoogleCalendarPage(self))
     self.AddUserStory(GooglePlusPage(self))
     self.AddUserStory(BlogspotPage(self))
     self.AddUserStory(WordpressPage(self))
     self.AddUserStory(FacebookPage(self))