Update perf page_set to use story_set

tools/perf/page_sets/tough_video_cases.py

Index: tools/perf/page_sets/tough_video_cases.py
diff --git a/tools/perf/page_sets/tough_video_cases.py b/tools/perf/page_sets/tough_video_cases.py
index 7c42d98a50b2308510133a01a772eae6e6f8c588..728501aa08d3e7e5d7644b21e19d68d20004489f 100644
--- a/tools/perf/page_sets/tough_video_cases.py
+++ b/tools/perf/page_sets/tough_video_cases.py
@@ -1,8 +1,8 @@
 # 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 ToughVideoCasesPage(page_module.Page):
@@ -498,14 +498,14 @@ class Page36(ToughVideoCasesPage):
   def RunPageInteractions(self, action_runner):
     self.SeekBeforeAndAfterPlayhead(action_runner)
 
-class ToughVideoCasesPageSet(page_set_module.PageSet):
+class ToughVideoCasesPageSet(story.StorySet):
 
   """
   Description: Video Stack Perf benchmark
   """
   def __init__(self):
     super(ToughVideoCasesPageSet, self).__init__(
-            bucket=page_set_module.PARTNER_BUCKET)
+            cloud_storage_bucket=story.PARTNER_BUCKET)
 
     self.AddUserStory(Page1(self))
     self.AddUserStory(Page2(self))