Update perf page_set to use story_set

tools/perf/page_sets/tough_animated_image_cases.py

Index: tools/perf/page_sets/tough_animated_image_cases.py
diff --git a/tools/perf/page_sets/tough_animated_image_cases.py b/tools/perf/page_sets/tough_animated_image_cases.py
index 733fa53852e3ae9697837fc71cd9e34853d20ad5..a27124780f95ea3a388a6b20a92c9be5bad8c82d 100644
--- a/tools/perf/page_sets/tough_animated_image_cases.py
+++ b/tools/perf/page_sets/tough_animated_image_cases.py
@@ -1,8 +1,9 @@
 # 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 telemetry import story
 from telemetry.page import page as page_module
-from telemetry.page import page_set as page_set_module
+
 
 class ToughAnimatedImageCasesPage(page_module.Page):
 
@@ -14,7 +15,7 @@ class ToughAnimatedImageCasesPage(page_module.Page):
     with action_runner.CreateInteraction('ToughAnimatedImage'):
       action_runner.Wait(10)
 
-class ToughAnimatedImageCasesPageSet(page_set_module.PageSet):
+class ToughAnimatedImageCasesPageSet(story.StorySet):
 
   """
   Description: A collection of difficult animated image tests