Update perf page_set to use story_set

tools/perf/page_sets/image_decoding_cases.py

Index: tools/perf/page_sets/image_decoding_cases.py
diff --git a/tools/perf/page_sets/image_decoding_cases.py b/tools/perf/page_sets/image_decoding_cases.py
index 335040f6332fd78ba4aa266e230f5aaaaba1987d..8065c820f5dece101e5359b14aadd12ba875f878 100644
--- a/tools/perf/page_sets/image_decoding_cases.py
+++ b/tools/perf/page_sets/image_decoding_cases.py
@@ -1,8 +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.page import page as page_module
-from telemetry.page import page_set as page_set_module
+
 
 class ImageDecodingCasesPage(page_module.Page):
 
@@ -13,7 +14,7 @@ class ImageDecodingCasesPage(page_module.Page):
     with action_runner.CreateInteraction('DecodeImage'):
       action_runner.Wait(5)
 
-class ImageDecodingCasesPageSet(page_set_module.PageSet):
+class ImageDecodingCasesPageSet(story.StorySet):
 
   """ A directed benchmark of accelerated jpeg image decoding performance """