Update perf page_set to use story_set

tools/perf/page_sets/tough_scrolling_cases.py

Index: tools/perf/page_sets/tough_scrolling_cases.py
diff --git a/tools/perf/page_sets/tough_scrolling_cases.py b/tools/perf/page_sets/tough_scrolling_cases.py
index 5076a858e96ab7b4a816f3680cf7ad463ebc376f..18dc5d7eaaa3afc02f2ce6edb129815a48183065 100644
--- a/tools/perf/page_sets/tough_scrolling_cases.py
+++ b/tools/perf/page_sets/tough_scrolling_cases.py
@@ -1,9 +1,9 @@
 # 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.internal.actions import page_action
 from telemetry.page import page as page_module
-from telemetry.page import page_set as page_set_module
 
 
 class ToughFastScrollingCasesPage(page_module.Page):
@@ -24,7 +24,7 @@ class ToughFastScrollingCasesPage(page_module.Page):
           speed_in_pixels_per_second=self.speed_in_pixels_per_second,
           synthetic_gesture_source=self.synthetic_gesture_source)
 
-class ToughScrollingCasesPageSet(page_set_module.PageSet):
+class ToughScrollingCasesPageSet(story.StorySet):
 
   """
   Description: A collection of difficult scrolling tests