Update perf page_set to use story_set

tools/perf/page_sets/tough_scheduling_cases.py

Index: tools/perf/page_sets/tough_scheduling_cases.py
diff --git a/tools/perf/page_sets/tough_scheduling_cases.py b/tools/perf/page_sets/tough_scheduling_cases.py
index 8a4251ad8ebe295d925541693ef5fc204393113a..cd8bbde8184920f7e8689ba7ce369daae5486068 100644
--- a/tools/perf/page_sets/tough_scheduling_cases.py
+++ b/tools/perf/page_sets/tough_scheduling_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
 from telemetry.page import shared_page_state
 
 
@@ -389,14 +389,14 @@ class SecondBatchJsPage(ToughSchedulingCasesPage):
       action_runner.WaitForJavaScriptCondition('window.__finished')
 
 
-class ToughSchedulingCasesPageSet(page_set_module.PageSet):
+class ToughSchedulingCasesPageSet(story.StorySet):
 
   """Tough scheduler latency test cases."""
 
   def __init__(self):
     super(ToughSchedulingCasesPageSet, self).__init__(
         archive_data_file='data/tough_scheduling_cases.json',
-        bucket=page_set_module.INTERNAL_BUCKET)
+        cloud_storage_bucket=story.INTERNAL_BUCKET)
 
     # Why: Simple scrolling baseline
     self.AddUserStory(ToughSchedulingCasesPage(